TwistMyMind has stated the normal way to redirect SMS messages; however as AT&T users we do not have that option. Even per RIM's documentation (as TwistMyMind has stated) the solution for this very problem is doing just that:
KB13465-"SIM card is full please delete SMS messages" appears on the BlackBerry smartphoneFor BlackBerry® Device Software 6.0, complete the following steps:
On the Home screen of the BlackBerry smartphone, open the Messages application.
Press the Menu key and click Options.
Click Text Messaging.
In the SMS Text area, clear the Leave Messages On SIM Card check box.
Press the Menu key and click Save.
This is a documented issue, and so far there has not been a suggested fix. I would suggest backup your device and reload the OS following this tutorial here:
All In One Guide - How To Upgrade Your Device - with Pictures & Videos
You can find the latest official release here:
BellMobility_9800_184.108.40.2060 - Downloads - BlackBerryOS.com
yes its a little bit of work, but it may be worth trying in the end. Hope this helps.