It seems to me (maybe incorrectly) that Celeste would need to do similarly to other clients in that the mail file needs to be broken up into multiple files, generally according to the mailbox scheme set up by the user.
It might turn out this way, but I'm hoping that you don't have to.
If you do have to split the database, then I prefer the way that John Maloney has suggested: divide by time. Have a 1999 database, a 2000 database, etc. I know that personally, I rarely retrieve mesages older than a few months back, but it's not so infrequent that I'll want to retrieve a message when I've forgotten what category it was in.
Incidentally, the UI isn't very helpful if you have multiple databases. You can do "Celeste openOn: 'mail2000'", but then each Celeste window will be independent.
I asked my wife the other day what it would take to get her away from Eudora. She's open to such a move but it would have to be reasonably feature equivalent and user friendly. She and my children are currently on Macs.
We definitely need more hacker types to play with it before Jo Blows are set loose on it--email is one of the last things on a computer that you want to be difficult! In particular, it would be great to have suggestions on just what non-techies would like to see.
Alan's suggestion of a side-by-side comparison with Eudora sounds like it would be enlightening.
Being able to import or use standard mbox format mailboxes would be great. I frequently join mailing lists which have either downloadable archives or archives available from the mail server. It would be nice for Celeste to handle that.
Celeste can read and append to Unix-format mailboxes, which Eudora also uses (or at least used to use). More formats would certainly be nice, though.
-Lex