I'm currently dual-booting Vista 32-bit and Windows 7 64-bit RC. Here's my Vista setup:
4GB RAM
C: drive - 500GB, Boot, holds C:\wow folder
D: drive - 500GB, used for writting FRAPS
E: drive - 160GB, holds E:\wow2 folder
F: drive - 160GB, holds F:\wow3 folder

I'm not symlinking anything, each drive contains full copy of WoW. All of my mods are in C:\wow, and nothing but Jamba and TooltipOnMouse for the other two.

Now when I chose to install Windows 7 64-bit, I just installed it straight to my 2nd drive (D: to Vista). Even though it installed to a separate physical drive, it still enabled the dual-boot menu (I was prepared to use one-time boot menu in BIOS to access the 2nd drive, but that wasn't necessary). Now here's what happened to the drives under Windows 7...
8GB RAM
C: drive - 500GB, Boot, used for writting FRAPS (D: drive under Vista)
D: drive - 160GB, holds D:\wow2 (E: under Vista)
E: drive - 160GB, holds E:\wow3 (F: under Vista)
...

Only 3 drives. Apparently Windows 7 tries to play things safe and refuses to automatically assign a drive letter to the Vista C: drive in order to prevent you from screwing up that OS. A quick trip into Disk Manager and I was able to assign F: to that drive...
F: drive - 500GB, holds F:\wow (C: under Vista)

As I use batch files for starting up my 3 copies of WoW with Maximizer, I just copied those batch files to the new desktop in Windows 7, edited them to update the drive letters, and off I went.