I had exactly the same problem with my screen freezing after updating to 10.4.2. I spent hours searching forums online (hard re-booting every 10-15 minutes after each freeze) for an answer. Last night I think I figured it out... (I haven't had a freeze since then - fingers crossed).
At first I thought it might be the 1gb Ram upgrade that I added to my new 12 inch powerbook 1.5GHz with superdrive that came installed with Tiger 10.4.1. Most forums seem to point to hardware problems as a cause for screen freezes. There was no rhyme or reason to each freeze. It wasn't specific to a particular application, so I was sure it must be a hardware issue.
Meanwhile, one of the many third party "widgets" I had installed during what I call widget-mania to describe the initial excitement with using this new feature had crashed unexpectedly so I decided to use Tiger's new widget manager to delete all third party widgets from lesser known sources. I also swapped the RAM upgrade with the original 256Mb stick and re-booted. No freezes. I worked on it for a while and no problems, I couldn't make it freeze no matter how many applications I switched between or used, opened or quit. So, I returned the 1Gb RAM stick back and I haven't had a freeze since. I will update if the situation changes, but for now I believe the widget is the culprit!