Not that there's any reason to think that it would have been otherwise, but I just wanted to confirm that this is still a problem in 3.1. Also, I've changed the title of this bug report from "Cannot specify custom trash folder using IMAP in Gmail" to "Cannot specify custom trash folder using IMAP". The "in Gmail" has always been misleading, because the problem occurs with any IMAP server, so I went ahead and removed it.
Not that there's any reason to think that it would have been otherwise, but I just wanted to confirm that this is still a problem in 3.1. Also, I've changed the title of this bug report from "Cannot specify custom trash folder using IMAP in Gmail" to "Cannot specify custom trash folder using IMAP". The "in Gmail" has always been misleading, because the problem occurs with any IMAP server, so I went ahead and removed it.