OS X :: Grey Dim Screen Swipes All The Way / You Need To Restart Your Computer

So i updated to 10.5.7 the other day and it took a few restarts to get it going, but all is well now.

i was just tooling around on my computer with several programs open as i usually do (safari looking at gmail, ical, mail, itunes, ichat, weatherdock, TM error logger), then all of a sudden a grey dim starts at the top of the screen and swipes all the way to the bottom then the following message appears: "You need to restart your computer. Hold down the power button for several seconds or press the Restart button."

Anyone else have this issue with 10.5.7? what did it mean if you had it in the past?

I'm running a mid 2007 MBP 2.4 SR, with stock 2GB RAM, stock 160GB hard drive, 2 Western Digital external hard drives, one USB for Times Machine and a FW800 for iTunes media.