Comment 18 for bug 153635

"We've all run into this bug one by one. Most have settled for the disable-roaming workaround. I tried this, and couldn't get it to work. I had to manually run: ifconfig eth0 up & dhclient. "

I can confirm that I've had this problem in VMware Fusion 1.1 and disabling roaming in Ubuntu's network preferences aswell as running ifconfig eth0 up and dhclient fixed the problem.