Just like the Samsung Galaxy S was easily rooted, hackers have also discovered a simple method to unlock the device and allow it to run on other carriers. This means that owners of the T-Mobile Vibrant can get their device working with an AT&T SIM card and the AT&T Captivate can be modified to work on T-Mobile.
Normally when you unlock a device to run on another network you can still get data services, but 3G speeds are not possible because the carriers use different frequencies for their high-speed networks. However, the T-Mobile Vibrant has a 3G radio that supports multiple frequencies, including those needed for AT&T 3G. Unfortunately, the Captivate does not support the AWS bands needed for T-Mobile 3G.
Instructions can be found in the source link and look fairly simple for those of you who have attempted any previous hacks. Samsung actually left the unlock codes on the phones and this hack uses a method to extract them and tells you how to enter them back in. Like always, use at your own risk.