Keep inmind that a fair amount of file locations and formats are likely to change for Tiger, and Mail is a prime example. The new features they're talking about, coupled with the fact that they're integrating SQLite (a public-domain embedded SQL database) into the OS, would seem to indicate that Mail will almost certainly start using it. That will probably change more than a few file formats, because while it's possible to bring other file formats into a SQLite database, the slowdown generally isn't worth the trouble unless you plan to only import once and then save in SQLite's own database format.