Thanks.
It would be surprisingly messy to add this functionality to the client. You would have to store a modified transaction list locally and perhaps back it up with your wallet.dat file every time you restored a backed-up wallet.
If I did not like certain transactions appearing I would send some coin to another wallet file and using that one to make these transactions that you don't want to see in your main wallet client. Eventually the client will allow several different wallets to be open at the same time so this action would be easier to manage.
Ha! There is a name for this, interesting.