As you may have read previously, I haven't taken the plunge on an iPhone yet (although I heard McNulty did the other day, which means I might be the only one left at TUAW). But I have been considering it a lot (more than is healthy, probably), and one of my considerations is email. Just a few months ago, I switched completely to Gmail, because I wanted one inbox for all my addresses that I could check and send from anywhere. And with an iPhone, I worry how reading messages will work-- I know I can log right into my Gmail account, but sometimes I have hundreds of messages, and I suspect things will get lost in the mix.
Catcubed had the same problem with his iPhone, and now has worked out what he calls the perfect solution. He's written up a tutorial to get Gmail, IMAP, and his iPhone all working together nicely. Basically, he's got it set up so that Gmail archives and checks everything, and then forwards messages that make it through the filter onto his iPhone via a special address that's IMAPped to Mail. Outgoing messages from the iPhone are synced back into Gmail (to be archived), and look like they're coming from the Gmail address. So while he's on the road, Cat can send and check directly from his email address via his iPhone, without having to wade through everything that comes in, and everything will still be kept in his Gmail when he gets back.
It sounds darn complicated to set up. But once it gets rolling, it sounds like I won't have any problem getting my iPhone email right. Now, I just need to come up with the $500 for my cell phone budget...