It's not good, but it could be worse.
I was unable to restore anything from the hard drive. However, it turns out that (a) mail from before I switched to OS X was unaffected (it wasn't associated with an "account"), and (b) my husband did not overwrite my external firewire drive, so I was able to import the inbox and sent mail from 12/02 through 6/20/03.
That means I lost more than three month's worth of mail, including a lot of stuff related to our grant (administrative, not data), and all the mail related to my three upcoming trips. I'll spend most of today trying to figure out how to reconstruct the most critical components--flight itineraries, hotel information, etc.
So, if you sent me mail that had critical information, or needed a reply in the past 3 months, it might be wise to resend it, or to at least check with me about whether I need another copy.
To prevent another such disaster, I've installed Norton Utilities (to allow for better recovery of erased files in the future), and will be regularly backing up my data to my external hard drive. I think that will be a weekly ritual now.