I also use the Belkin, it's a great case for a reason not yet mentioned. If you look at the picture of it closely you will notice that the leather at the bottom is more supple than the leather that protects the front and back, and what I do is I put the iphone in the case, inside my pocket, clipped to my pocket edge. When the phone rings I push up on the bottom supple leather, (through my pants) which folds into the case and pushes the phone out of the case, then I easily grab it. After a few weeks of use it's a little tiny bit looser, but still holds tight. Love the case, just got it until the iskin came out, but after reading the reviews I'm sticking with the belkin.
If I've confused you please see this pic for the "push up" technique. I don't clip to my belt though, I'm too clumsy. The clip is secured to the case though and is pretty dang tight (which is good). Also there is some sort of a steel plate reinforcing the exterior leather side. Ssee the belkin logo on the case? That is steel and it goes farther out under the leather and makes the outer leather very very strong!
