Here's a video I recorded of the Samsung Vibrant with "death grip" issue. It's important to say that while there was a noticeable decrease of connection speed, I could NOT drop a call even if when it went from low signal of 2 bars to 0. The quality of the call wasn't even affected either. So I wouldn't say this is a serious problem as with the iPhone..
YouTube - Samsung Vibrant Death Grip issue
YouTube - Samsung Vibrant Death Grip issue