Today, however, Mail started crashing when I try to display the current CNET message. I then tried to view yesterday's CNET message in the Trash, and Mail crashed again. On relaunch, Mail was trying to display yesterday's message in the trash, and crashing before I could do anything. This happened once last week, and I had to restore ~/Library/Mail/ from my nightly backup. Today, I went looking for another solution.
I tried relaunching Mail again, but this time, I held down the Shift key as it launched. To my surprise, Mail opened with no mailbox selected, and everything seemed to be OK after that.
[robg adds: As far as I can tell, this isn't documented, but it works perfectly -- and is much better than the solution I used when I had a 'crasher' email in my inbox. In that case, I used the Terminal to edit the Mail files directly and remove the offending message.]

