The 7650 didn't support them, and the 6600 either. I got so fed up with Nokia's broken IMAP client I ordered a D500 instead.
Obviously that was a mistake as the phone is nowhere near as capable as a Symbian one and was quickly returned.
I am still wondering if with the advanced 6630 they have finally allowed you to view mail that has been sorted server side (THE WHOLE REASON PEOPLE USE IMAP!!!) into sub folders.
Yeah it works perfectly, I have 3 different mail boxes defined and can download all my mail, even deleting an item on the phone goes back to the server as do Read/Unread.
At least on my 6680 I can now connect to an IMAP4 server and subscribe to and see all subfolders under the main folder on the IMAP4 server. The 6630 should work, too, as it is based on the same generation of Series 60 software (I just haven't tried it myself).