Task switching on iOS 4.3.1?
I played around with it for a few minutes, and eventually it went back to the old mode where task switching works.  I have no idea why, and I didn't change any setting.

It's not just the speed: I've never had trouble getting task switching to work, and for the first 10 minutes after upgrading, it didn't work no matter how fast I clicked it.  It would zip over to the useless search screen and back really fast.