10.5: Mail with Gmail IMAP

I've got my Gmail setup as IMAP in Mail and all folders like trash etc are setup in Mail except when I try and set up the Spam folder from Gmail as Mail's Junk folder it doesn't do anything, I select Spam then go to Message > Use this Mailbox for > Junk, and nothing happens.

Any ideas?

No one else has had this?

Well as no one seemed to know, after rooting around I figured out it was just the option "When junk mail arrives: Move it to the Junk mailbox" within Mail prefs. For some reason mine was set to "Mark as junk mail but leave in my inbox".

Thanks