Same problem I just went through 2 months ago on a 3g. I upgraded my old 3g to 3.0, immediately wifi no longer worked. Took phone into apple, they replaced due to faulty wifi. Problem at that time was that I had a friend take it in for me, he never paid attention and they took my 16 gig iphone and replaced it with an 8 gig.

I was pissed but there was absolutely no way for me to prove it was a 16 before. even tech support on the phone said I had an 8 before when I gave him my old phones serial number. But I know for a fact I didnt because when i went to restore my iphone, i was about 3 gigs shy of all my data fitting again.

Anyways, I decided to live with it, figured since the 3GS was out, and all that was available for a 3G at the new pricing was the 8gig, so there were no more 16 gigs to replace it with.

Now just tonight, I notice my wifi isnt working again! I installed 3.1 about a week ago, and all has been working fine since then.
I have tried resetting the network settings, as well as resetting the entire phone and not restoring, starting fresh, nothing works, but the data network does work.

I'm currently out of the country, and wont be back into the US till mid November, so I have to wait until then to take the phone in, at which point I'm sure they will replace it again.

My only thing this time is I would like to prove to apple that my original iphone was a 16gig, yet dont know any way of doing so. I plan on going in if i can prove it, tell them the screwed me over with a smaller phone, and now I want a 3GS to replace my phone.

Any chance of that happening? If I gave you guys the serial number of the old phone any chance anyone can look it up for me anywhere?
If i try the warranty page, it says the phone has been turned in for a warranty exchange, serial 81833RAHY7H

Anyways, sorry for the long post, long time reader, first time poster. Had to vent somewhere.