We don't know what it'll be, but Palm is teasing an announcement for later today. As stated on the official Palm blog:
"check back later today (or follow palm_inc on Twitter) for an update direct from the Web 2.0 Expo in San Francisco."
Palm assures us that this isn't some kind of lame April Fools. As such, we're hoping for a release date (by country) and price or at the very least a free, general release of the Mojo SDK. Whatever it is, you'll find the details here just as soon as they are announced.