Well, for what it's worth, I jailbroke my iPhone 4 when it was running iOS 4.1 and installed MiWi (the only way you could tether via wi-fi prior to 4.3). It worked FLAWLESSLY. For the 3+ months I was using MiWi to tether, it never dropped. I primarily use it to stream internet radio in my car using my iPhone 4 and an iPod touch. (as an aside, Apple should consider allowing MiWi on the app store, since it's a fantastic product!!)
I upgraded to iOS 4.3 a couple of days ago and changed my plan with AT&T to include the tethering option. I am now 100% a victim of the same thing you guys are all complaining about. I can connect to my iPhone 4 from the iPod Touch and it will stream for 6-8 minutes, and then the connection is dropped and, unless I restart the Personal Hotspot on the iPhone 4, it won't connect again. You'd think that the Apple solution for wi-fi tethering would be better than one intended for jailbroken iPhones...think again!
I'm experiencing the identical issues when I connect with my laptop, or my older iPhone 3GS. In all instances, it will connect for a few minutes, and then drop the connection.
So this only leads me to conclude that this is
ENTIRELY a software issue and not a hardware one. Since I was able to use the same hardware flawlessly before, and now it fails 95%+ of the time.
I'm sure you'll be hearing TONS of these complains and Apple will be forced to come out with some sort of hotfix for this. I'm convinced (because of my own experience) that this is a software issue.