I've had the Mail app on OS X Lion crash a few times on me lately.
No idea why. But at the same time I seem to have been experiencing a lot of corrupt attachments. One PDF came through part scrambled, another was readable one day and not the next. Since the mail is actually housed on Gmail, I thought I'd check the attachments in the web interface.
They were just fine. So I decided to clear out the cache OS X Mail keeps, and force it to re-download all my messages, and their attachments.
This is simple enough to do. Go into mail preferences and the Accounts section. Click the Advanced tab of the mailbox in question, and choose "Don't keep copies of any messages" in the "..offline viewing" drop down.
Just to be sure, I closed mail and re-opened it. Messages would now only appear if I specifically clicked on them. So, went back into preferences and set the offline viewing setting to "All messages and their attachments".
Now everything works fine again. Bizarre.