If you are one of those unlucky people who still don’t have IMAP support (and other neat brand new features) in GMail, try to change you language to US English. For me, it did the trick in every account I own that were not in english.
Switching back to your preferred language, downgrades the Gmail version as well, what a pity… well, I think it’s only a question of time. Meanwhile, if IMAP support is vital for you, this works quite well.
The eternal fight between admins and computers 