I remember reading about this once before. A little Googling and I found it:
http://64.233.169.104/search?q=cache...gl=us&ie=UTF-8
Apparently VMWare specifically prevents Time Machine from backing up its disk images, because for some reason it will lead to data corruption of Time Machine runs while VMWare is active. (Don't ask me why!) There are some work-arounds on that page I linked, but perhaps it's not a good idea to follow them if it could lead to data corruption...