Check to see if the default gateway IP address is missing from the IP settings on the target machines. Also, you might lose the default gateway address after every reboot, until the problem is corrected.
To fix this problem manually on a single machine:
1. Open the registry with regedit.exe
2. Go to the path: HKLM/System/CurrentControlSet/Services/Tcpip/Parameters/Interfaces/
3. Here you'll need to select the CLSID of your networkadapter where you change the settings. You'll probably recognize it by looking at the value IPAddress that will have the IP of this adapter.
4. Open the value "DefaultGateway" by doubleclicking it. You will see a list of all the gateways that disappeared! (maybe in your case it will be only one, in my case there were due to lots of testing several addresses) Very likely, the first line will be empty. I'm not sure, how this little flaw can make such a big problem. However, if you manually remove this first empty line, click OK and reboot your system everything should be working fine again.
However, next time you re-image you will get this problem again until Microsoft fixes this problem. You can request a hotfix for 2008 and Vista. I am not sure about Windows 7.
http://support.microsoft.com/kb/973243
http://support.microsoft.com/kb/299357
Also, make sure you have the latest Altiris SP and agents.