you dont need to be connected to 3G or WIFI continuously....i never saw a reason...here i am 17 months into Eris ownership and my battery from day one still gets 48 - 72 hours of life off one charge...why?
I dont stay continuously connected to the internet...
I keep a WIFI and Mobile shortcut on my home screen and simply turn it on and off in a matter of miliseconds when I need to do something online; be it check emails, surf, tweet
FB etc... when i am done, finger the shortcut (man, that sounds dirty

and turn off the online access...
remember, task killer app, check that icon several times a day to kill anything that has auto-turned itself on in the background...handscent SMS, native SMS still a battery drainer...
anyhoo, those are all the goodies i can offer you...same ones i did way back when...
3 more months, $150 credit towards a Thunderbolt and I will say that this Eris I have has been a fault-free pleasure...
holla