I retrurned mine yesterday, at the same store where purchased. When asked why, I said there is a known GPS issue with the Galaxy S line, and does not work as advertised. Further, on AT&T wireless' own website, it states it has voice dialing, which it does not.
The salesman tried to exchange it (to save his commission no doubt), and I refused, stating why would I want another of the same phone with the same issues.
After requesting to speak to a manager, and re-stating the same issues, he waived the restocking fee. Whether he felt I was right or not, I think he knew I was not leaving without a full refund.
Once I was home, I called Customer Care to make sure I was back to where I started before the Captivate: my unlimited data was still in effect, and my contract date & upgrade eligibilty was reset.