The surprise could be a revamped MacBook Air. At least, that's what Daring Fireball thinks:
A "sneak preview of the next major version of Mac OS X", and, I'm just guessing here, the brand-new way-cooler MacBook Air.
The other update that is long overdue is iLife. However, it seems unlikely that they will introduce a new Mac OS X (which will take at least one hour to present), a new "way-cooler" computer, and iLife. We'll find out a week from today.