Right now the top 2 problems confronting users of VMware Fusion are the following:
- No network connection after upgrading Fusion 2 and Windows XP (1016466)
- Troubleshooting Fusion 3 network issues in a Windows 7 virtual machine (1017565)
The second one is specific to problems with a Windows 7 VMs, the first deals with changes in the network connection handling between Fusion 2 and Fusion 3 and changes in network drivers between Windows XP and later versions.
If you are running into networking issues with Fusion lately, definitely check out these two articles, they can save you a lot of time and frustration.