Bioware have, on their official forum, explained how the DRM for Dragon Age II will work. Buy the game on Steam, it simply uses Steam - in other words, you can play offline as long as you log in online and make sure to connect at least once every couple of days. Business as usual there.
The retail version won't use any form of disc check, which means you can play it without the disc in your drive. It will come with unlimited installs, but you'll have to log in during installation to show that you're the owner. Also, your copy will only allow five different PCs to play it during a 24 hour period. You will be able to play offline, but just like in the Steam-case you'll have to verify your copy by connecting to EA's server every couple of days.