This happens to me all the time when I'm using Facetime on my iphone 4 (jailbroken on 4.3.3) - the call will drop and my phone displays 3G instead of wifi. I even have 3G Unrestrictor installed, which I thought would keep the facetime call going even if my phone did drop from wifi to 3G for some reason. I guess I'm wrong about that though since the call drops every time my wifi connection drops. I have no idea why my phone drops the wifi connection, especially since I usually sit in one place the entire time I'm using Facetime and there's no way it's out of range of my Time Capsule. Apple support has been unable to help me with the problem too, so I've been dealing with it the entire time I've had the phone - very frustrating.
Didnt happen often
but its now happening on two iPhone 4's and iPod touch 4 G running iOS 5 beta 2, did not happen with beta 1.
Two ipads are working fine on it
What kind of router is it? Recently my Linksys died, ended up buying a Netgear. My iPhone 4 on 4.3.3 started randomly dropping the wifi connection, but other iphones and laptops didn't. Restarted phone, deleted & recreated connection, restarted, etc, etc.
Finally had the router check for a firmware update, there WAS one, which seems to have fixed the problem.