Not sure if this will help, but a little known fact is that you are to wait until the Communication Manager loads & states "no device found" THEN insert your card. ......as per my call to AT&T "experts". Good Luck!
Go into device manager (in vista go to control panel (classic view) and open device manager. On XP go to start -> run and type in devmgmt.msc then click on okay). Then in device manager look at the bottom where you see universial serial bus controlers, click on the +. then right click on "Sierra Wireless Aircard 881 Device". A menu will pop up. Left click on uninstall. Then click okay. (If there is a check box that comes up make sure that you DO NOT check it). When the "Sierra Wireless Aircard 881 Device." goes away. Take the card out and restart the computer. DO NOT put the card back in untill computer boots back up and communication manager says "No wireless device detected". Put the card back in and then in the bottom right you should see it reinstall the drivers.
If you look into device manager and there are no drivers anywhere that say sierra wireless in the beginning. Call ATT.
If that doesn't work. Open Communication manager go to tools -> Settings -> Select the GSM tab -> make sure device selection is set to auto.