While one can only speculate on the reasons for the block, some have pointed out that it most likely has to do with the Bandargate scandal. Mahmood has been quite outspoken about the issue on his weblog, and has also started the much commended Just Bahraini campaign.
Hang in there, Mahmood! If anything, this will all come to pass.
Note: Fortunately, mahmood.tv is still accessible from Bahrain-based ISPs as alyousif.tv
Note II: As of 2 November, 2006, mahmood.tv has been unblocked.