Should I virtualize Windows on Mac OS X using Parallels Desktop or VMware Fusion?
I was in the exact situation as you 2 weeks ago. I have developer friends who have used Parallels in the past for their Windows development VMs, and they had since moved to VMware Fusion 3.

I started off with a trial of Parallels Desktop 6; in a nutshell, I had issues virtualizing some of my physical machines, but overall it ran my VMs well. However, I did find the Coherence mode a little cumbersome in the UI. There were too many shortcuts created for my various VMs and the start menu used to launch Windows apps felt clunky.

So...I moved on to a trial of VMware Fusion 3. Despite all the performance reviews that say Parallels kills Fusion, running a VM in Fusion feels smooth and polished and starting, suspending and resuming a virtual machine are all very fast operations thanks to the SSD of the Macbook Air. The product overall is high quality and requires very little resources - I run 1 to 2 VMs for several hours at once during development. On my 4GB Macbook Air I only ever use half of that memory, including the operating system. (I have set my development VMs to use 1GB of RAM each).

And those problems I had virtualizing my physical hardware with Parallels? Gone with VMware. All my hardware was virtualized on the first attempt, and VMware even flawlessly imported the Parallels VMs I had been working with.

So, I licensed VMware Fusion 3 and you can even get a  rebate on it like I did. That was the clincher for me.

I'm happily using VMware Fusion 3 every day to run my Windows 7 development box with Visual Studio 2010, Oracle 10g Express, SQL Server 2008 Express, and other tools installed.