Onboard wifi can be a funny beast. The hardware compatibility list forums & BackTrack wiki were a big help to me when getting started. Personally, I've always had bad luck with Broadcom chipsets. I own a Hawking HWUG1A & Alfa AWUS036H...both are USB wifi. The Hawking will go into monitor mode fine, but packet injection is hit-and-miss (mostly miss). The Alfa card will do monitor mode & packet injection no problem, out-of-the-box.
For maximum compatibility with BackTrack, I'd go with the Alfa AWUS036H, and install BT in dual-boot. I believe the BT developers recommend the Alfa AWUS036H for the Offensive Security Wireless Attacks course.