I had "Android System" eating my battery too. I'm stock/rooted. I had Cerberus installed briefly, uninstalled - about when my phone started being hot (bottom third) when sleeping in my pocket. Drove me nuts - sorry, but I simply cant tolerate an IDLE phone getting hot in my pocket! Anyway, I rebooted the phone, turned it off, pulled the battery, installed Better Battery Stats - eventually the phone stopped having massive drainage from "Android System" and now it's not hot anymore whether it's on browsing the web or idle.
I guess there are two points to my post 1) the way I could tell my problem was fixed was the phone stopped getting hot -and- 2) the problem hasn't come back for me after a simple fix of multiple-power cycles. I've seen mixed results with a fair number of people saying that rebooting their phone fixed there problem and about an equal number saying the problem either wasn't fixed by rebooting or it came back again later.
If it happened to me again I would open a Terminal and run the "top" command and just keep watching it until I figured out what was eating up my battery (CPU time).
EDIT: Oh - and to confuse things more I used Titanium Backup (pro) to freeze/un-install some of the T-mobile bloatware in addition to setting email to sync slightly less often...