Although you are able to add Exchange ActiveSync, IMAP, POP, CalDav, and CardDav accounts, it does not seem as though you will be able to invite BlackBerry Exchange Server or BlackBerry Internet Server accounts to the party.
To put it in a nutshell for the blurry-eyed, RIM might throw in native email support to the PlayBook, but it might actually leave BES or BIS support out. Ah well, it should not be any huge shock or surprise there, considering prior rumors pointed out that the QNX-powered BlackBerry Colt will not feature BES support due to the difficulties involved in getting it to work on QNX.