Erm I just accidentally sent Bitcoins to myself. It currently has 18 confirmations and the client actually says 'Payment to yourself'.
Couldnt someone make blocks very very large by doing it repeatedly?
It would take a few seconds to make a endless bash loop doing it.
Why can you send coins to yourself?
You could create the same problem with two clients and a script running on each. The real question that concerns me is: What happens if someone creates software to create trillions of transactions? If older transactions are not removed from the transaction chain, then it appears there is a denial of service flaw.