I just noticed this occuring on my Evo 4g last night. Been noticing my battery dieing a little faster than normal, so I opened up Battery Monitor last night and saw that under Partial Wake Usage, Google Voice was listed as having been running for 6 hours and 58 minutes. I had not received any GV SMS's or used the GV app at all yesterday. I then checked under Network Usage in Battery History and it said that Google Voice had received 10MB of data since last unplugged (10,2xx,xxx) or something like that. Not sure what it was transferring since I hadn't used it at all.
Ended up uninstalling Google Voice last night before bed. Charged phone all night, upon waking up, shut phone off (still on charger).. booted phone.. then unplugged.. The battery has been doing a lot better today so far. I may try to reinstall tonight and see how the phone acts tomorrow.