I have the exact same problem on a 3G iphone. I initially did an update and the phone ran ok - no overheating, no speed issues, no battery life problems - but I was getting a lot of instability issues with apps crashing. So I did a clean install and added all my apps back in from scratch.
Since then the phone has been running alarmingly hot, the battery life is pitiful and apps are sluggish or have jerky frame rates.
After reading this post -
http://discussions.apple.com/thread.jspa?threadID=2045240&start=30&tstart=0
I have tried all sorts of combinations - wifi on/off - push on/off etc
Push is definitely the culprit.
I can turn everything on - wifi, 3G, location etc - and everything runs fine. As soon as I switch push back on the phone runs slow, games are jerky, phone runs hot and the battery bar practically disappears before my eyes.
The above post recommends deleting the push account, restarting and setting the push account back up again. I tried this and it didnt make any difference - but it might work for some of you.
I'm now left with paying for mobileme and using fetch settings - which pretty much forfeits the point of having it.