It looks like I’m one of the lucky ones whose IMAP support has already been turned on in gmail. Others are reporting that it hasn’t shown up for them yet. Settings now has a “Forwarding and POP/IMAP” tab with an “IMAP Access” section at the bottom. Just select Enable IMAP and you’re good to go. Mine was already enabled when I checked, so I jumped straight to configuring iPhone access. Inbox, All Mail, Drafts, Sent Mail, Spam, Starred, and Trash all show up as IMAP folders as do all of your labels. Very nice.