Purple-ish Plaid screen on startup

Start up using the system disk that came with your computer. i.e. start up normally and insert the disk,then do a hard shutdown (hold the power button down until the machine shuts down). Wait about 10 seconds. Turn the power on while holding down the "C" key. When it starts up select "Disk Utility" from the file menu. In the window that opens select you HD and click repair disk. Come back to us and let us know the results of that procedure.

This suggestion is good but didn't work for my PB pro 17" which displayed the same symptoms.
Resetting PRAM and PMU had no effect, either. Couldn't get at a log.

It would not boot the Hardware test. Here is my workaround: I started the PB in terminal mode and tethered it to an older laptop. I crossed my fingers and ran the previous version (OS X - 10.4) of Disk Utilitiy > Disk repair. Magically, under the old routine, the plaid screen gradually erased itself (I had feared a video card or motherboard issue).

Mac hardware seems to be so reliable that my 2 previous laptops are still fully functional. I use one for travel and the other for backup and/or guests at home. I also use Time Machine so that events like this one no longer throw me into a panic. I was kinda looking forward to having an excuse to buy a newer model, yet I'm relieved that it won't be necessary now! I hope this experience is helpful to others.