If for some reason the domain name servers (DNS) are not responding, but the network connection is up (for example, if the domain name servers are down), NetInfo takes ages to authenticate a user (probably because it tries to look up something and there is a long timeout delay).
The problem shows up, for example, when trying to de-activate the screensaver. The problem is made much worse by the fact that the screensaver seems to have a glitch which removes the password screen right after it is done showing the spinning ball for a few minutes, making it practically impossible to deactivate.
The solution is simple: unplug the network cable, and within a few seconds everything is back to normal. I do not know if Apple is aware of this bug.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020603092951219