I haven't verified this yet, but I wondered if it's a possible bug?
I bought a Dell XPS M1330 over a year ago and it came preloaded with Vista Home Premium. Since I want to keep the batteries in good condition I run the laptop with no battery connected and AC plugged in whenever it sits on my desk at home.
However, in Windows 7 it seems that Windows wants to put the laptop to sleep after 10-15 minutes if I do this. Since I have various USB devices plugged in, especially external storage, this is a problem. It goes away if I run with the battery plugged in but like I said, not verified yet.