I had a machine image that was configured to work on Microsoft Virtual Server. Since I'm trying to switch to VMWare Workstation instead of Virtual PC, I thought I'd use the VMWare Workstation's built in ability to open or convert the image.
Unfortunately, when I went to do so, it complained of some parameters that it didn't understand and exited. So, I took the VHD and created a Virtual PC VMC for it, using the information I found in the original VMC. After saving the VMC, I was able to open it with no problems in VMWare. Evidently VMWare doesn't understand all the options that Virtual Server can put in the VMC.
Something to try if you run into the same problem.