I think that has to do with the IP lease with the DHCP host. I don't remember having this problem in pre-10.0.4, but what I do now, is go into System Prefs -> Network, change to manual IP, hit Save, change back to DHCP, hit Save, and it all works again.
Basically, you're restarting the netinfo database. There's probably a way to restart it from the command line by sending it some kind of HUP signal or something, but I don't know how to do it. Anyone?
-Rob