I have no idea what "I'm Fucking Ben Affleck" is...
Okay, quick/short history for you, and why his appearance was so funny, if you care to check them out...
(Funnier even, cause I'd only heard of him as an Operatic Pianist/singer as well)
Jimmy Kimmel (Late Night Host) always did a stupid joke at the end of his show saying "My apologies to Matt Damon, We ran out of time" Which was to say that Damon wasnt quite important enough to actually make it on the show.
Sarah Silverman (Kimmel's longtime Girlfriend) and Matt Damon made a video entitled "I'm Fucking Matt Damon", with Sarah singing the lead...
Kimmel's rebuttal was "I'm Fucking Ben Affleck", citing that if Matt Damon could have sex with the person most important to him, he (Jimmy) could do the same.
Kimmel went ALL OUT though, getting heaps of celebrities to sing about Kimmel's homosexual relationship with Affleck. I won't give them all away but you'll see heaps of people in it (to the point where I'm very surprised you hadn't heard of it)
I'm talking We Are The World type love fest.
Well worth looking up just for Groban's "He's Fucking Beeeeeeeeeeeeeeeen, He's Fucking Beeeeen Affleeeeeeeck" line. Not to mention all the other cameos
Sarah's Video...
http://www.youtube.com/watch?v=4QXnHtNSkD4&feature=related (says uncensored, but its not, was made for US Tv so all versions are censored)
Jimmy's Rebuttal...
http://www.youtube.com/watch?v=j_pFTAY7MF8&feature=related