The BBFC classification for Mass Effect has revealed that the game has been awarded a 12 rating for "moderate violence and one sex scene".
Violence is pretty common these days, but a sex scene is sure to cause a bit of a stir. According to the extended classification over on the BBFC website the scene in question is "brief and undetailed" but does include "breast nudity in one version of the scene".
http://www.pro-g.co.uk/news/18-09-2007-6482.html