this same thing happens to me in the same scenario. Has the poster figured this out yet? It is extremely annoying and hinders my ability to video chat on my iphone while in my home near my router. I will start a video call on facetime, skype, oovoo, or tango, and after 10 or so seconds the quality becomes very poor then the phone switches to 3g.
Facts:
Verizon Iphone 4 latest SW update.
Lynksis wireless G router.
speedtest 14mbps average.
ipad works with skype, facetime anywhere in the house.
aaron