i believe it does use lossless saving. but then again if you mess something up, iphoto does keep the original images stuffed away in your iphoto library folder.
And if you want to get back the original, just select the picture you want, and go into the "file" menu and click on "Revert to Original". Very cool, very simple.
I am not so much concerned about "reverting to the original". That's certainly a nice feature but I am mainly concerned about being able to save the newly cropped file without recompressing (since recompressing would result in quality loss). I am not a Mac user yet so I can't veryify this first hand. Does anyone know for sure whether this is possible?