I finally took the plunge a few weeks back, and upgraded my Mac Book Pro hard drive (requiring a week with the Apple repair center to do the upgrade!), in preparation for replacing the Windows XP Boot Camp partition with a Windows 7 installation (with Windows XP moved from Boot Camp to a Parallels VM).
The transition was fiddly; I used Parallels Transporter to copy the Boot Camp XP to a virtual machine image on an external USB drive (which I then copied to the Mac Hard Drive). I had to use a USB drive, as I couldn't get Parallels Transporter to work over my LAN. Once I'd proved that the VM started OK (and was backed up to my RAID drive!), I then got rid of the boot camp partition, and tried to install Windows 7 in a larger Boot Camp partition. However... I found was that the Mac drive was too fragmented to resize the Boot Camp partition (!).
I did some reading around the subject, and was surprised to find that Macs don't have a defragmenter option...! My solution was not much fun, but was easier than it sounds: I cloned-off the hard drive using the incredible Super Duper! to another spare USB drive, double-checked that I could boot off that drive; then cloned-back the Super-Dupered drive back to replace the contents of my Mac Book drive. That fixed the fragmentation, so I could finally configure the Boot Camp partition as I wanted and proceed with installing Windows 7.
I chose a 64-bit install of Windows 7 so that I could use Mixtikl and Noatikl VST plug-ins in 64-bit mode under Windows.
Using 64-bit so far hasn't caused me too many problems; Beyond Compare (which is still isn't available in a 64-bit version) did't integrate at first with Windows Explorer. However, thanks to a comment from Aaron from Scooter Soft (see below), I found the configuration option that let me re-configuration as a shell extension, which now works perfectly!
There is also a 64-bit version of Tortise SVN which integrates with Windows Explorer OK. There is also a new version of Cygwin that works fine with Windows 7, and I found a 64-bit copy of GVim that integrates with Windows Explorer.
All in all, Windows 7 is quite a bit faster than XP (though still not as fast as Mac OS!), and is in general easier to use now that I've customised Windows Explorer to work more like that in Windows XP. After a week of use, XP already seems dated and I have no intention of going back to it other than for occasional software testing! I also love the way that out-of-the-box, Windows 7 gave me a read-only view of the HFS partition on my Mac.
So, what about a Windows 7 virtual desktop manager (Mac has the wonderful spaces built-in)? Well, simply install VirtuaWin - it works beautifully, and is open source!
Finally, you'll need Cygwin to get a greate scripting environment, which lets you share scripts between Windows and Mac.
That said, there are far more tools for Windows than for Mac... so I am happy to use both Windows and Mac, mixing-and-matching where required. Where would I be without Beyond Compare and Tortoise SVN on the Windows side, for example? :) The combined Mac OS X/Windows/Parallels/Bootcamp system really is a good one for code development...