I have been doing some checking, and have found no evidence to lead us to believe that a Tribune of the Plebs was exempt from criminal prosecution or even arrest. The Tribune's sacrosanctity, as has been previously stated, seems to only extend to physical harm. Each Pleb was bound by a solemn pledge to kill any person who harmed a Tribune during his term of office. Beyond that, the sacrosanctity seems to not apply.

Regardless, it stands to reason that if the Tribunes were indeed granted exemption from criminal trial, the Romans would have made it abundantly clear in the myriad historical records we possess. It would appear they have not. The office holds no Imperium, we see no evidence of a Tribune using his powers to escape prosecution, and we see no law explicitly granting this power to the Tribunes. Volusian has asked that common sense prevail in the discussion...Well, I am not certain that common sense would allow us to begin assigning various powers to various offices without explicate historical evidence supporting these changes. Doing so would not only be terribly foolish and most likely incorrect, it would also set the worst of precedents.

That's my two cents, anyway.