Another problem I encountered after upgrading to 3.1 was my visual voicemail stopped working. And, as it turns out, the hack I had used (under 3.0) to allow tethering was what was causing all of my problems. When I contacted AT&T about the visual voicemail issue, they elevated me to level 2 tech support and that person specifically asked me if I had installed any sort of hacks to the phone. I sheepishly admitted I had and they told me the only way to restore things to proper working order was to do a restore and select "New iPhone" instead of restoring from a backup. I decided to do it. It was a giant pain the ass but I managed to get all of my songs/photos/podcasts and applications reinstalled and get all of my settings restored. It took HOURS (hey, I'm retired so I have plenty of time). And, when I was done, visual voicemail worked properly and my data problems were gone. The iPhone is more responsive too (it grew somewhat sluggish after installing 3.1).
I still believe I'm getting somewhat reduced battery life (vs. 3.0) but, after the full restore, it seems better than it was just after installing 3.1.
I do not plan to install any further hacks to my iPhone. I'll wait for legal tethering and, if the price is right for the amount I'll use it, I'll give AT&T some extra money each month. But, if the price isn't right... well, don't tell them about this little app called NetShare that I downloaded a year or so ago! 🙂
Mark