i've seen little evidence of it myself.
more intelligent denial-of-service attacks are not difficult. for example, off the top of my head, you could send many transactions with invalid signatures, tying up the cpu of peers.
attacks just lead to an arms race between the developers and the attackers. in principle they should be easy to address, but principle is not always the same as practice. in practice, i'm not sure what would happen in the event of coordinated distributed attacks, in part because developing for bitcoin has a learning curve greater than for most similar projects; there are maybe a dozen of us who could respond quickly. and the work might be asymmetric: it might be easier to mount attacks than to develop countermeasures, practically speaking.
because people here are loathe to find any fault with bitcoin and routinely impugn the motives of those who point them out, i feel like i should directly quote satoshi on this, from his last message in the forum:
As Gavin and I have said clearly before, the software is not at all resistant to DoS attack. [A recent change] is one improvement, but there are still more ways to attack than I can count.
