I had the same problem : a good carbon copy of my 10.3.9 System on a Firewire HD;
a clean install of Tiger on my iBook-G4
and migration nights.
Mail 2 would only import empty folder out of my old ~/library/mail folder !
at the same time, any Mailboxe creation (+) was stopped by an error message !
AppleHelp says it is a permission problem,
so I corrected a copy of the old Mail-folder autorisations, with getInfo and 'apply inside' button, and filled Read & Write for user, group and other
it did not help.
But a simple Solution is to remove only 1 file :
On the Tiger drive, go to /users/~userName/Library/Mail/ ;
Remove the file : Mailboxes ; which is a file at that time, and not a folder as in Jaguar.
Restart Mail (2.0) to import your Mail !
it works.
NB : you still may have to proceed one mailboxe at a time;
and use the "Import : Other" option rather than ": previous Mail app."
I first saw this tip at <Mail 2.0 [Archive] - AppleInsider> :
<http://forums.appleinsider.com/archive/index.php/t-53558.html>
Now that everybody is waiting for Leopard, it's a late tip anyway.
Procast