Some application screens won't redraw in MBP
Fixed it!

I installed the pending update for Safari, which I thought was quite irrelevant, and that has sorted it out. Initially I thought that this did some system 'tidying up', which kicked a graphics module back into shape. But a colleague points out that Mail may well use WebKit's rendering engine, and it is possible that all of the IRC clients I tried also do.

The problem is still most strange, as the update for Safari has been pending for a great deal longer than the problem, which must have cropped up around ten days ago or less. So it was working without that update, and furthermore I've not idea what I did to trigger the fault.

Still, sorted now. Hopefully this post will be useful for anyone who encounters a similar problem.

Lightbulb moment: I seem to remember that I used the Software Update feature to download but not install the Safari update (5.1.2) between Mail working and breaking. Since the update required a restart to install, I left it and forgot about it. My guess is that, although it appeared to just download the Safari update, in fact it modified (and broke) the current configuration. If I can find a way to feedback to Bad Apple (tm), then I'll do so. Feedback done!