Here's an article about the behavior I reported in the OP:

Bugs & Fixes: Safari web content maladies

Safari 5.1.2, released today, apparently fixes this blank-page issue. A 10.6 or a 10.7 version is available for download. Things seems fine so far in my limited use.

So far, so good here as well...

The update said it also fixed an issue relating to memory usage.

Both Safari and the Web Content process seem to be using much less memory now, which is excellent news.

Now, not so good. Every morning I open 10 web sites in tabs. This morning, the following two were completely blank:

http://hints.macworld.com/headlines.php

http://www.newyorker.com/online/blogs/hendrikhertzberg/

and the formatting was completely broken on this one:

http://www.tuaw.com/

All three pages displayed correctly after manually reloading.

But then, while writing this post I went back to look at each of 7 still-open tabs. All were completely blank when I first clicked on them. I had to wait five or ten seconds for each page to be redrawn.

I should note that Time Machine was backing up during this time with mdworker (and mdworker32?) doing their cpu-hogging deeds.