I have the same problem with my MC and Vista!
I have mapped 4 folders on my main PC's D: drive for sharing, and put media on them. Even data is on my PC, I access it through mapped Network drives.
Whe MC starts, it connects to these NW drives, and this creates problem of Vista not entering the Sleep mode.
I was changing the System Configuration (msconfig), and diabled Security Accounts Manager. As sharing and mapping of drives doesn't work anymore, my Vista sleeps like baby. Juts the problem is now that you can't run Vista witH SAM disabled, it it does't share and map resources!!!
Interesting enogh, my other Vista media player PC, which is accessing the same media on the same mapped drives of my main PC, doesn't have any problem with sleeping!
So far conlusion: MC is having conflict with Security Accounts Mangaer, when it's accessing shared/ mapped network drives on the same Vista PC. This conflict creates issues of Vista Sleep.
Matt, with this information, you should be able to replicate this problem on your test systems.
Any ideas, how to fix it?