I am afraid their surprise has already been spoiled. They will announce the immediate availability of the PRS-300, PRS-600 and the covers (regular and w/light), as well as eBookLib 3.0 with Mac support, and (possibly) FW upgrades for PRS-505 and/or PRS-700. But we already knew that, didn't we?
I doubt there will be anything else. I can hope they will also pre-announce the large-screen reader, and some connected (Wi-Fi or GSM SIM-card) readers. On the other hand, those are clearly not ready for release, so what's the point in announcing something before its time, and killing the sales of what's available.