I'm running Ubuntu 8.04 under VirtualBox on Vista. I've created a virtual network interface for the vm and bridged my network connections. The network connection was down for a while, so when I started vm there was no connection. After a while however, the connection was restored and I was able to use it on Vista. The problem is, the vm didn't feel that the connection was restored. I googled for that and got a useful thread on ubuntuforums. I applied the solution I found there and added a launcher to my panel.
Right-click the panel -> "Add to Panel..." -> "Custom Application Launcher" then enter:
Type: ApplicationClick OK.
Command: gksudo /etc/init.d/networking restart
Comment: Repair Network Connection
The launcher should appear on your panel. When you click the icon, it'll run the command you entered and will ask for your password. Hopefully, this should 'repair' your network connection.
btw, this is my first post from ubuntu!