Not 100% confirmed, but it sounds about 98% there. Deshawn Stevenson is also going to Dallas, with Mav filler (Q. Ross or Tim Thomas) going to Washington.
From Adrian Wojnarowski's Twitter:
Wizards near completion of trade that sends Caron Butler and DeShawn Stevenson to Dallas for Josh Howard and one other player, source says.
11 minutes ago from UberTwitterThe Wiz and Mavs still discussing whether Quinton Ross or Tim Thomas will go Washington with Howard, league source says.
5 minutes ago from UberTwitterYahoo reported overnight that trade was in advanced talks but Dallas backed away for a bit. Mavs returned to today to get it done.
3 minutes ago from UberTwitter