I heard on Ryan Seacrest's morning show that Neil Patrick Harris is guest judging on American Idol today at the Dallas auditions. This was also confirmed by one of Seacrest's staffers, Negative Natalie, on Twitter. Joe Jonas was yesterday's guest judge.
NPH is no stranger to Idol. He has been in the audience in the past and even got a "Doogie" shoutout during the Nick Mitchell/Norman Gentle Group 2 performance of "And I Am Telling You I'm Not Going". He is also a good singer in his own right, having performed on Broadway (Cabaret, Stephen Sondheim's Assassins). He also played Mark on Rent in LA and San Diego and starred in the internet musical "Dr. Horrible's Sing-Along Blog".
Here he is in a musical number at this year's Tony Awards: