I remember Paul Obrien tweeting that the GSII was really open, seemed to actively encourage modding. Twitter search is horibble so I can't find the tweet now though.
Just so you know, twitter's search only goes back a limited amount of time because its targeted at real-time results, not historical. Google with site:twitter.com might get better results.
AFAIK The SGII is open, but I had heard rumors a while back from people I trusted that Samsung was headed in a similar direction. There's no reason to believe it, and with HTC taking a stand I'm hopeful.
Meanwhile, Motorola vaguely semi-committed months ago and nothing has happened.
I don't really buy that carriers are demanding this as Samsung currently doesn't do this.