I'm going to go ahead and preempt all the AT&T haters who will inevitably reply to this post with "HAHAHA AT&T SUCKS ONLY 800MHz!!!11eleven"
The hardware in this video was admittedly "several months old". Several months means this hardware was built before the international Galaxy-S was even released. It's VERY EARLY NON-PRODUCTION HARDWARE. The Captivate WILL run at 1GHz.
Thank you, we now return you to your regularly scheduled AT&T bashing.