I'm running a Win7 64bit based Fusion 3.1.3 VM on the MBAir. The VM has 1.5 Gigs of Ram and a 32 GB virtual hard disk and that works just fine. I run the VM concurrently with Entourage, Safari, iTunes, and occasionally other programs, and the MBAir is ecrypted with FileVault2. I've yet to run across performance issues - these i5/i7 processors and the SSD hard drive are fantastic. While I do have a BootCamp Partition, the VM doesn't access it.
I did try Fusion 4 but quickly went back to 3 as I encountered too many issues; I'm waiting with 4 until I find they've been resoved.