Ok, scratch my IMAP IDLE Kaiten question, seems it's enabled by default.
Ran my own test. Sent a few test messages to a gmail.com account, all were received as unread in Gmail, AquaMail, Kaiten.
Waited a bit, set them "read" in Gmail web mail... waited for Kaiten to update, it did not... not even after its own scheduled mail check (the countdown to next mail check in the title bar).
What am I doing wrong, how can I see Kaiten updating "unread" message flags for Gmail?