I'm unable to alt-tab away from the DDO client to another application without hanging the client, as does anything else where the client loses mouse focus (running in windowed mode and clicking on the menu bar or launcher, KDE notificatons running, etc).
I'm on Fedora 17 using KDE most of the time (also happens under the new Gnome, XFCE, and other desktop environments). I have a nVidia card. My steam games and GOG games all work without hanging when losing focus, so I suspect it's a specific DDO dll or configuration option.
This is a game that almost requires web browsing in the backgroud, as nearly any useful reference information is in a wiki found outside the game client, so this is a particularly pernicious problem. I have to either ask my guild members to look things up for me or close the client, web browse, reopen the client, log into my character, and hope all this is done within 15 minutes or I'm kicked out of my adventure and have to start over.
If anyone knows how to access the DDO wiki from within the client that would be a great substitute until I can get a real fix. I can access the store, the forums (athough search doesn't seem to be working right) and MyDDO. I can't find the wiki.