As usual, Apple's press invitation is cryptic—it says only to "Come see what's new for the Mac on October 20, including a sneak peek of the next major version of Mac OS X." Above that text is what appears to be a Magic TrackPad with a lion hidden behind an opening Apple logo.
So. What does it mean? Using my Sherlock Holmes-like powers of deduction, I'll venture some guesses.
1. This is called a "sneak peek," so no new version of OS X will go on sale immediately after the announcement. I'd be surprised to see it on sale this year at all.
2. There's a picture of a lion behind the apple logo—Apple likes naming its OS updates after big cats. This would be OS 10.7, the follow-on to Snow Leopard. Rumors about Lion have been floating around all year—apparently Apple has been testing it in-house, so websites can tell when Lion-equipped computers visit online.
3. It's probably no coincidence that Apple featured the Magic TrackPad in the invitation to this event. Touch probably figures prominently into the new OS's feature set.
4. The surprise here could be updates to iWork '10 and iLife '10, software suites for productivity and for managing photos, video and more on the Mac. If Apple announces those—and I personally think it's likely—they're more likely to go on sale in time for the holiday season.
For investors, the upshot is that if iWork looks impressive and hits store shelves in time for the holidays, there could be margin upside for Apple, since software is very profitable. Most of the Apple analysts certainly have an iWork update priced into their models—but if this update is particularly exciting, it might cause them to get even more optimistic than they have been of late.