[frumiousirc]

What stops attackers from poisoning the DHT? Could one publish false name <--> IP address associations?

[ycmbntrthrwaway]

Cryptography. Identity is not simply a name. You don't choose it but generate.

[magila]

The crypto would stop you from actually impersonating the target, but you could easily DOS someone by flooding the DHT with bogus data for their id.

Edit: You could also get the IPs of all the people trying to contact the target.