I also had the battery drain and hot iPhone 2G running OS 3.0 (updated).
Disabling Push did the trick, but a guy just posted a possible solution which makes sense in a way.
http://discussions.apple.com/thread.jspa?threadID=2063246&tstart=0
{quote:title=xingkai wrote:}Go to "settings" and hit "general" then "restore all settings."
Your iphone will momentarily pause and undergo some changes as all the settings are changed to default. After switching all your settings back, you may change them to as you like. For some reason doing this fixes the battery drain. I think it's because some 2.2.1 settings were not carried over in 3.0 and so you must set everything back to default to kind of "refresh" the system. {quote}
I restored using iTunes, but then I put back my settings and apps through iTunes, so the settings were from my old iPhone 2G. I think that can also be the problem with the 3G and 3GS (new 3GS users shouldn't have this problem since they have no old profile they transfered).
I'll try and see if this works and post back.