You are diverting your attention away from your attacker to pick up your weapon, EVEN if you are on the ground. Just like you are diverting your attention away from attacking someone to attack their weapon, you suffer an AoO, unless you have Sunder. So, unless you have "Improved pick things up", you STILL provoke an AoO. The rules do NOT say any different.
If you say otherwise for your campaign, that's cool, but it IS a house rule.
See, that's my ENITRE point, is that *nowhere* in the rules does it say, that if you are prone, you get to pick something up for free. The rules just flat out say that picking something up is a MEA that provokes an AoO. I wanted to see if there were any *stated rules* that I was missing. You're just coming up with House Rules, which is fine and all. I'm just trying to make a point.