Considering that "Late Night with Jimmy Fallon" just debuted on March 2, it seems a little ridiculous that the show is going to be taking its second week off already. The first rerun week was the week of April 6-10, when the other NBC late night shows were off as well. But next week, Jay Leno will be doing first-run shows.
Looking at the keyword searches that led people to this site, "why is Jimmy Fallon in reruns already" skyrocketed to the top of the charts during the first week in April. I suspect it'll happen again next week.
Mo 4/27: Regis Philbin, Kat Von D, Jon Glaser, Morrissey (R 3/23/09)
Tu 4/28: Jason Segel, the Amazing Kreskin, Dan Rollman & Corey Henderson (R 3/20/09)
We 4/29: Glenn Close, Kristen Wiig, Gomez (R 3/31/09)
Th 4/30: Tina Fey, Jon Bon Jovi, Santigold (R 3/3/09)
Fr 5/1: Lauren Graham, Judah Friedlander, Cold War Kids (R 4/3/09)