Seconded. Samsung has supported 3rd party developers since the launch of the phone, with a full-fledged CM7 ROM out already, and most likely a MIUI port in the near future.
When the (codenamed) Nexus Prime releases, that will be the phone to get. The Nexus S (4G or not) is still a fantastic phone, and will run smoother than a majority of the dual-core phones on the market today.
oh yes..Nexus is pretty awesome. for me it was a toss up between S II and the Nexus. but the Dual Core bowled me over plus the fact that i found that the S II booted a bit faster than the Nexus and other bits and pieces here and there and ofcourse S II is a lot sleeker.