I ordered my iPhone 4s also. I'm using the 3GS-16GB right now and the AC app runs very well. I haven't noticed any significant amount of memory usage with the AC app. I really don't think it's an issue.
One complaint with the AC app: When I use it to look up a patient's phone number, it would be so nice to simply touch the phone number within the AC app to dial the number, rather than memorizing the number (or jotting it down). As of now, you have to close the AC app, then physically dial the number (somewhat of a PITA.)
