Multiple sources over the last year have stated they are writing it including Fox All Access. As for the time-line, keep in mind that the IWTB greenlight was announced in late October 2007 and filming began on December 10th 2007. Filming wrapped on March 15th 2008 and it was released on July 25th 2008. So it's not impossible and it's happened for "The X-Files" before.
There has been a credit crisis since then, and movie funding has been hard to come by. Look to more recent films and you'll see a much longer lead-in time.