As for the US, according to the same source, Verizon will start selling its version of the Galaxy S II sometime in July. A specific date is unknown right now, and may have not even been set yet, but a Verizon spokesperson did confirm the July time frame.
It's also very possible that the story of the Galaxy S II will be very similar in the US to that of the original Galaxy S, with each carrier getting its own version and its own brand name to go with it. We've heard before that Verizon's Galaxy S II will be called Function, AT&T's will be called Attain, and Sprint's will be the Samsung Galaxy Within. The naming scheme has not yet been officially confirmed by either carrier or Samsung.
We'll have to wait and see if the Galaxy S II manages to outsell its predecessor (which has apparently been sold in 14 million units so far), but it certainly looks like it could. It's got the specs too, being one of the devices to pack as much as is possible into a smartphone form factor.
source: Unwired View