Lyxen
Great Old One
Barring any distractions, being clearly seen makes it impossible to hide just as much in combat than out of combat so it's not any harder. In fact it could be even easier in combat as you have more chances to be distracted!
Yes and no, as you are also more focussed during combat. Actually, the game makes the assumption that, if anything, you are very alert during combat: "In combat, most creatures stay alert for signs of danger all around,..."
Anyway, as most of the discussions around stealth, there cannot be any definitive ruling, it depends too much on the situation. This is what JC clearly and rightly points out in the podcast on stealth "This more than almost any other part of the game, is going to rely on the Dungeon Master."
People looking for a single, clear ruling like "you can hide" in combat or you "cannot hide in combat" are going to be disappointed. It's as simple and complex as the first sentence on hiding "The DM decides when circumstances are appropriate for hiding."
So if the DM decides that it's not possible to hide in combat, here you are, you can't, at that table for that particular game.
Personally, I allow it, I just play my characters and adversaries in a manner befitting their experience and intelligence. So yes, a guard might be surprised by a halfling hiding behind his half-orc friend the first time, but unless he is a complete idiot, he will not necessarily be fooled a second time when there is nowhere else for the halfling to hide. It's all circumstantial, even more than most areas of the game.