FYI, I had been using the Marware Premiere for my iPhone 3G and 3GS for 2 years. I have both the original magnetic version and the new velco version, here are some comments:
Both cases fit the iPhone 4 great, especially when using a slim body cover, I am using the new iFrogz LuxeLean on my iPhone 4 and the fit in my old Marware Premiere's is near perfect.
The magnetic version is waaay better than the velcro version. Unfortunately Marware stopped making the magnetic version when the 3GS came out. My magnetic version is nearing the end of it's life, the leather covering the belt clip is worn thru and the stitching is coming loose. The magnets did affect the compass on the 3GS, the result being that I had to recalibrate to use the compass application. This didn't really bother me because I hardly ever used the compass anyway.
The velcro patch on the new version is weak and too small. The top flap doesn't sit flush like the magnetic version does and consequently I was always catching the lip when sitting down, etc. It drove me nuts and caused me to stick with my deteriorating magnetic version and live with the compass recalibrations.