I am not claiming that the network is impervious to DoS attack. I think most P2P networks can be DoS attacked in numerous ways. (On a side note, I read that the record companies would like to DoS all the file sharing networks, but they don't want to break the anti-hacking/anti-abuse laws.)
If we started getting DoS attacked with loads of wasted transactions back and forth, you would need to start paying a 0.01 minimum transaction fee. 0.1.5 actually had an option to set that, but I took it out to reduce confusion. Free transactions are nice and we can keep it that way if people don't abuse them.
That brings up the question: if there was a minimum 0.01 fee for each transaction, should we automatically add the fee if it's just the minimum 0.01? It would be awfully annoying to ask each time. If you have 50.00 and send 10.00, the recipient would get 10.00 and you'd have 39.99 left. I think it should just add it automatically. It's trivial compared to the fees many other types of services add automatically.