I will definitely let you know how my testing goes.. I ran the same iPhoto test on my wife's 2 year old Macbook and it does the same thing, which tells me it is probably performance related with the mini...
As I think about this, it should not be a big deal as a little stutter with presenting full screen images is not a dealbreaker, right? The problem is if it is having a very minor problem NOW with smooth transitions of photos, how will it run in 1-2 years with new versions of iphoto, more resource intensive software, higher pixel images, etc... I am getting the feeling that if I invest in the current mini I am getting a graphics card that will get me by now versus a graphics card that more than I need now and is ready for the next 3 years...
I will do some additional testing (hopefully this weekend) and provide an update...