Note: this entry has moved.Longhorn doesn't work in current VMWare Workstation product (4.0). However, you can use the latest beta (4.5 RC1) which adds (experimental) support for Longhorn under a Windows 2003 Server VM. I've also downloaded it to fix a corruption I got on my VM disks after running Partition Magic, and it worked flawlessly.
Download it from this location using the following registration information:
(information taken from here - not that I know japanese, but I figured it out ;))
Your current VMWare licence key will work. Enjoy!