Spotlight in Leopard has some UI limitations;search results display only the three fields "Name", "Kind" and "Last Opened", and you can't add more columns. Mail.app works around these in two ways when it indexes mail: 1. The "Name" field shows both the message subject and the sender; the sender is in black text, while the subject is in gray. 2. The "Last Opened" field is munged to show the actual date of the e-mail, rather than the last time you opened it. I didn't realize this until I tried letting Thunderbird index the same IMAP folder as Mail.app. Thunderbird's results, while technically more correct, aren't nearly as useful, and I've already gone back to letting Mail.app index my mail; I couldn't find anything with Thunderbird. Relatedly, Mail.app shows up as "Mail message", which is a built-in Kind that you can filter on; "Thunderbird metadata" is not. I'd like to see TB be a drop-in replacement for Mail.app in Spotlight.
As of 3.0.3, 'Kind' now displays 'Thunderbird Mail Message' chris