I think a tab for Sent and a tab for Received DMs on the main UI would be acceptable.

Settings could have options for timed auto-refresh, notifications for mentions / @ replies (use IM LED/vibe pattern).

I agree that profile changing abilities might not be as important, as users don't do it that often, and it's not a real requirement in a mobile client.

Twitpic and plixi support would be great, but I hope it's not inline - maybe automatically loaded and cached upon recognition but only viewable after the specific tweet is selected.

Reply-to-all would also be a very nice addition.