Interesting....thanks ;-)There are a lot more differences than just the front mechanical button which everyone talks about.
#1: the i9100 has a built-in FM stereo RDS radio
#2: the i9100 has three front buttons one of which is mechanical
#3: the i9100 is factory unlocked (it was NEVER locked to start with)
#4: the i9100 has slightly thinner & smaller housing dimensions
#5: the i9100 weighs a few grams less
Here are the size/weight specs for each:
Samsung Galaxy SGH i777: Dimensions 126 x 66 x 8.9 mm Weight 121.9 g
Samsung Galaxy S II i9100: Dimensions 125.3 x 66.1 x 8.5 mm Weight 116 g
Most likely there are other slight differences which I have not yet investigated.
Check it out yourself at GSMARENA on their website.