I had a similar problem to OP (with Gmail set to retrieve every 20 minutes) ... BUT

Gmail wasn't the problem. I turned off 3G, uninstalled Battery Icon and stopped some sort of indexing (found in another thread). THAT fixed the problem (one of those three or a combination) - down to 8-20 mAh idle and I can use the phone a whole day without having to recharge.

Do you have Battery Icon installed?