I tried doing everything they said in every forum I could find with issues upgrading to 4.3.3 wifi connectivity problems. I recently upgraded my unlocked 3GS to 4.3.3 (because I accidentely screwed up my icons on 4.2.1 - icons went missing!) and everything worked great for about 24 hours.
The next day I wake up and find my wifi shows connected, but I have NO connection on wifi whatsoever. I did every reset known to man, including a full iphone reset (EVERYTHING GONE) but still no luck. I tried manually inputting some information into the STATIC IP section (even though I have DHCP, still nothing.
So I found my working old 3G phone, and tested connection...worked fine!! That phone is still on 4.2.1. So...here's what I did.
I checked the wifi info on the 3G and compared it with my 3GS. The IP addy was totally "effed up" on the 3GS. I changed all the info again on my 3GS to reflect what my 3G had...and VOILA!!! it somehow has worked...not sure how long it will work, since its only been 20 minutes since tested, but my hopes are high until apple fixes this issue.
Here's a quick snapshot of what I changed:
Wifi -> chose my network ->changed to STATIC -> filled in this info, which can be found in your router info:
IP Address: 192.168.X.X
Subnet Mask: 255.255.255.0
Router: 192.168.X.X
DNS: 192.168.X.X
After I saved this, I disabled the wifi on the old 3G just to make sure that my router wasn't maxxed out on the number of devices, plus avoiding IP conflicts.
I know this was long winded, but I hope it can be of some use to you guys out there. I tried EVERYTHING else except throw my phone out the window.