The Pronto shares the same $20 monthly plan as the original Peek, but on top of unlimited push emailing, that plan also supports unlimited text messaging. The Pronto's software is also a bit more advanced, including the ability to check out documents and PDFs, as opposed to just pictures.
The Pronto is available now, but unlike the original, it's only in black (they call it gray). [Peek]
UPDATE: Over on the Peek messageboards, one user claims that the old Peek will receive the same software soon. Consider that unconfirmed, but it seems likely enough.
UPDATE 2: Future Peek Classics will no longer have SMS.