What heada says is true, however exchange comes with the exmerge utility, to extract mailboxes from exhange, (converting them into .pst files). Tivoli have made some scripts , which need some small alterations, and you can extract all mailboxes to .pst's, and back them up as files. I know, it's a hazzle, an you need alot of extra disk-space, but it is possible. Haven't tried it yet, but I will at some point.
I believe it is worth thinking about, because it could be a greater hazzle to have an Exchange server standing by, to use for restores, and a large Exchange server takes a long time to restore, and you then have to use exmerge anyways, if you want to get just a single mailbox out of the restored exchange.
I know there exists som kind of API for this, because Veritas Backup Exec can do this in their last version.
May this enlighten your path, and lead you to the Solution. :-o
Rgds.
Geirr