Probably not related to the upgrade to 5.0. I had this problem with my iPhone 3G about 4 months ago. Power would drain in about 20-30 minutes, even when not being used. It's rare, but it happens. I fixed mine by doing a complete wipe and factory reset. Worked beautifully and I've been back to normal since. Make sure you do a full backup of all your stuff beforehand because everything (apps, photos, contacts, emails, settings) will be deleted.