Never done a HDMI splitter, but I once did a LVDS splitter for several LCD screens.
Even though the display resolution was moderate (XGA 1024 x 768), the problem described by Crutschow, of properly dealing with multiple differential digital signals in excess of 100 Mhz proved to be a very tough nut to crack.
Without a proper analyzer ( $$$) to measure skew and jitter, the color bleed would look horrible. In some instances the video would completely loose sync.
We had to ensure that every single signal path to be as identical as possible. Took several iterations of expensive boards.
Not a project for the faint of heart or budget minded.