I use two email accounts on my Pre: gmail and an IMAP account. I've always had decent (all day) battery life with both accounts set to "Get Email as Items Arrive".
Last week I changed the provider of my IMAP account and suddenly I was seeing MASSIVE battery drain - maybe 5-10% per hour on standby, it wouldn't even last through the day and this has never happened before.
I've now changed my new IMAP account to "Get Email every 15 minutes" and I'm back up to 1-2% per hour drain on standby.
Conclusion: all IMAP providers are not the same. I know both my old and new IMAP providers support IDLE but there must be something odd about my new provider that causes the battery to drain massively when trying to retrieve messages "as they arrive".
So, if you're having huge battery drain, tweak your email settings. On my old provider battery life was actually BETTER when on "Get Email as Items Arrive" but on my new provider it totally kills the battery.