Download Squad reported that GMail at long last supports IMAP. I just checked one of my Google-hosted domains, and lo! it is there.
Interestingly, they specifically address the iPhone in their configuration instructions. As a note, you must set up Gmail as IMAP, and not per the stock Gmail settings on the iPhone. “(Note: don’t tap Gmail; if you do, you’ll set up POP rather than IMAP. We are aware of this issue and are currently working to make setting up Gmail more intuitive for the iPhone.)”