Thanks for the input everybody. I'm not quite willing to shell out for Magic of Faerun just for the duelling rules, though.
Dwarven Godfather's description sounds similar to what I've come up with myself. If you're interested, here's what I've got:
Spellcraft Duel
When noble warriors wish to settle a matter of honor, they meet in a designated dueling field and have at each other with weapons. Usually such formal duels are to "first blood," and there is usually an empowered priest on hand to provide healing. Thus such duels rarely result in death.
When two noble spellcasters find themselves in the same situation, they will often engage in a a "Duel of Spellcraft." Such duels are much less predictable than the mundane kind, but follow many of the same procedures. Once the time of the duel has been set, the duelists' seconds meet to work out the details. This includes the location of the duel, what weapons or magic items will be allowed (usually none), and the features of the dueling ground.
The two combatants meet at the appointed time and place, along with their seconds. The seconds are charged with ensuring that the proprieties are observed by both parties and to stand in for the duelist should he be unable to begin for some reason. The proceedings are generally officiated by the highest ranking noble available (other than the duel participants of course).
The Dueling Ground
The dueling ground can take any form mutually agreed-upon by the participants. By default it is a flat, clear area 50 feet in diameter, marked out by flags and ropes (as such it is usually an octagon or hexagon rather than an actual circle). The dueling area is considered to extend 50 feet up as well. In the arcane academies of [[Ilium]] the dueling grounds can become quite large and elaborate, as each side seeks to add features to their advantage. There have even been cases of duels taking place within the great Coliseum late at night, with the grounds radically modified through powerful magic and illusion.
Beginning the Duel
At the beginning of the duel, the duelists must be free of any magical auras and be carrying only those items agreed upon by the seconds in the pre-duel negotiations. The two participants must submit to examination by the official or his designated aide. This includes a magical examination to ensure that no prohibited magic items or spells are being brought in. Once the official is staisfied, the duelists take up their positions on opposite sides of the dueling area. The official stands to one side and holds a brightly-colored silk cloth in the air. He drops the cloth and when it touches the ground the duel begins.
In game terms this means that both participants get to Ready a Standard Action for when the scarf hits. Assuming neither waits, conduct an opposed check to see who actually goes first. Each participant can use either their regular Initiative modifier (for physical actions), their Spellcraft modifier (for casting) or an appropriate ability check for other actions. If the results of these checks are the same, actions are simultaneous. Whatever order is determined here is used as initiative for the rest of the duel.
Rules of the Duel
While the exact rules of a given duel are worked out by the seconds, in the vast majority of cases, the following rules apply.
1) The duel ends when one of the following happens:
a) First Blood is drawn. In game terms this means one of the combatants has his Hit Points reduced below his Constitution
b) One combatant is unable to continue by virtue of being rendered unconscious or otherwise helpless for 30 seconds. Note that during this time, the opponent is not required to leave the incapacitated foe alone, though all the normal dueling rules apply. It's not unusual for the active duelist to pummel the opponent during this time, or cast additional spells on him. 30 seconds is 5 rounds
c) A combatant surrenders (including by leaving the dueling area under his own power)
2) Spells and other abilities that are likely to cause the instant death of the opponent are forbidden. Use of such spells constitutes attempted murder in most cases.
No Disintegrate or Finger of Death, thank you
3) Spells that bind or otherwise restrict an opponent do not count toward the 30-second limit unless they render the foe truly helpless. Thus Web would not count, but Sleep would. This is at the discretion of the official, but generally if the combatant is able to respond with a word and a gesture, he is not considered helpless (since he can cast).
4) Summoning spells are allowed, but the caster is responsible for the actions of the summoned creature.
5) Leaving the dueling ground of your own volition, whether magically or simply by walking is considered a forfeit. Being thrown from the ground by your opponent does not end the duel unless you are incapacitated at the same time. If the evicted duelist is able to continue, the duel is simply re-started from the beginning. Summoned creatures and effects are dispelled before the re-start.
6) By default, physical attacks are not prohibited in a spellcraft duel. It is not unheard-of for wizards of small magical talent but large stature to win duels by simply rushing their opponents and pummelling them into the ground. This is often considered bad form, though and prohibitions against physical attacks are a fairly common stipulation.
7) Spells that sap the life force of the opponent are considered too unpredictable and therefore forbidden.
No spells that do Con damage or drain levels. Spells that damage other abilities or apply negative levels without risk of them becoming permanent are OK
8) Spells that permanently impair the opponent are forbidden.
No spells that cause ability drain. Ability damage (except for Con) is fine, but not permanent drain.
Nonlethal duels
Many duels go beyond even "first-blood" rules to require that no serious harm be done to the opponent. This is especially common at the great academy, where a whole student culture of dueling is tolerated by the administration as long as it doesn't get out of hand. In this case the following additional rules apply.
9) All spells that do serious harm are prohibited. If the combatant is able to alter the spell to do non-lethal damage, it is allowed, but otherwise not.
No spells that cause HP damage.
10) Summoned creatures usually are not smart enough to do non-lethal damage. If this can be arranged in some way, then summoning is allowed, otherwise it is forbidden.
