I think it would be GREAT if the observed value (when applicable) could be included in the email alert.
Status: Error (Red)
Alarm: Guest disk space
Time: 10/9/2010 1:27:31 PM
VM disk free min space is below 5%
Observed Value: 4.25%
(I added the last line!)
It would be even better if the data used to compute the observed value were also presented. In this case that would be the size of the disk, the amount of free space, and the drive letter affected.
I realize the 2nd part of the request would take a lot more development time, but the first part should be no more than a few lines of code!