How about a delete button next to each message? To delete multiple messages, the user would fall back to the current interface.
Pro: Single click for a common operation. (Was: click + long move + click + drag + click.)
Con: Uses up a lot of screen real estate.
If the above takes up too much screen real estate, the drop box could be replaced with
Move selected to: [ Inbox ] [ Archive ] [ Deleted ] Pro: Removes a click+drag, an operation requiring a fair bit of precision.
Pro: The options are not hidden in a drop box.
Con: Not forward compabtible if support for more folders is added.
For simple operations, you just type the message in the chatterbox. You don't have to go to the message inbox. Of course, that kind of "simple operation" also has its disadvantage: the username and the "/msg " characters count towards the 256 character limit in that case.