I want to use this for a provably fair betting system.
SatoshiDICE uses the transaction ID to determine the lucky number but the reason it doesn't matter if it is random is because the transaction ID is just part of the input used to get the results, with the remainder kept secret at the time the bet is placed.
Another "provably fair" service, BitLotto (whose operator has since cut and run with the last month's worth of winnings) used the results of an external event (a state-run lottery) that occurred after the betting deadline as its apporach to offering provably fair.
But as others mentioned, the Trx ID is the result of the contents of a transaction, and thus can be manipulated.