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Here's the skinny: Though installing a new ROM usually resets your battery stats, it's not doing a reliable reset of stats. Most likely you don't have you phone plugged in to power when you're installing, you don't really have a full charge from the outset, etc. Letting it drain all the way down from this state and recharging WILL NOT HELP YOU, in the long run.
You need to be methodical in your approach, my friends. You mustn't skip steps (not implying that you were, but I definitely was).
For the record, we'll be referring to this bit on the wiki. Now, what's important is this:
Charge your phone. All the way. Top that sucker off. Then, with the power still connected (<- that's the important bit), reboot into recovery and wipe the battery stats. Once that is done, boot back to your normal phone, and once it's back up, remove the power cable. NOW! let it drain all the way down. Until it shuts off. Annoying little flashy red light doesn't count. Hell, I even let mine shut off, gave it a bit, turned it back on and ran it until it shut off again. After that, charge your phone all the way up again. You should be back to normal battery usage. I'm getting my standard day-and-a-half out of it, at this point.
In summation:
1. Charge phone completely, leave plugged into power
2. Boot into recovery
3. Wipe battery stats
4. Reboot to normal
5. Remove power cable
6. Drain that sucker all the way
7. Recharge fully
8. Rejoice!
MOD EDIT: Reason for drain given at: http://droidbasement.com/db-blog/, under 'Known issues' for Dream/Sapphire 32B kernel.
Edited by Skrki, 25 December 2010 - 03:35 AM.
Added information
battery drain seems to be quite well now
