At a convention of television critics during the summer of 2005, executive producer Chuck Lorre confirmed that the show's opening tune is performed by studio musicians and not by actors Charlie Sheen, Jon Cryer and Angus T. Jones (who merely lip-sync the lyrics). One of the actual crooners is singer and voice-over artist Elizabeth Daily. -source: IMDB.com, "trivia" section for "Two and A Half Men."
From "Trivia for Two and a Half Men" on imdb.com:
"...the show's opening tune is performed by studio musicians and not by actors Charlie Sheen, Jon Cryer and Angus T. Jones (who merely lip-sync the lyrics). One of the actual crooners is singer and voice-over artist E.G. Daily."
E.G. Daily (Elizabeth Daily) is probably best know as the voice of "Tommy Pickles" on "Rugrats" and "Buttercup" on "The Powerpuff Girls".
