There was a lot more childish humour in Blackader, and with house he seems to much more adult because the comedy in it is so much darker. i think he is more sexy now because he seems to have grown up... :-) (Hugh Laurie = Yum!!!)
Hehe, that's a constant matter of disagreement among the members of the German "House" message board - the question if he only became so damn sexy as Dr. House of if he was even before he took that role (the beard seems to play an important role in this matter). Let's say, percentage is about 50/50. IMHO: he is attractive, an he was before, but usually wasn't allowed to show that in his roles.
I remember reading a magazine interview with him before House started, where the female interviewer records her astonishment at how sexy he is in real life "until he opens his mouth" (when he immediately starts pulling "golly gosh" faces and speaking like Bertie Wooster). So I think the answer is that he's sexy unless he's being Hugh Laurie.