TNEF is an MS-proprietary format. When an email comes in with a Winmail.dat attachment, the router is smart enough (as of 7.0.2) to on the fly convert it to a usable format.
The IMAP task isn't as smart (yet). If you're running Outlook as an IMAP client (don't ask), there are common situations where it'll post winmail.dat files to the IMAP server. This obviously causes issues with webmail and other clients accessing the mail file.
Please smarten up the IMAP server!