I have been suffering with this problem on my Toshiba Tablet m200 for quite some time, and finally took the initiative to research the issue. At first I thought I was the only one dealing with this so I figured it was hardware specific, but when I asked around I found others with the same problem.
I didn't have an exact repro, but periodically when I closed the lid on my machine and left it alone for a few minutes and plugged in or took out the AC power, the screen would go black and there was no way to bring it back without hard booting the machine. I could tell that the machine was operating fine (the HD was spinning, the lights were on, you could hear the new mail notification in Outlook, etc) but the screen would not come back.
My machine was not set to go into standby when the lid was closed, but it was set to go into standby after 10 minutes. But often I was able to reproduce this in just a minute or two - I'd be using my laptop in a meeting, close the lid and walk back to my office, and couldn't get it back. Of course, this user scenario was also when I'd be changing to or from AC power.
I was pointed at the registry key mentioned in this KB, and after some testing this morning, it appears to have fixed the problem. I am told it is also fixed in XP SP2, although I haven't upgraded yet to verify that.