I have mine in my orginal iphone Vaja Ivolution Grip case. It fits great with no problems at all. After a couple of days the back of the case molded more to the curve of the iphone 3G.
Based on how they introduced new designs last year, I expect they will have more to offer for the 3G in the near future. Last year, they only started out with maybe 2-3 designs for the iPhone, but over the months following, they came out with more choices.