It’s expected that they will be rolling out updated iMacs (including updated designs) with the possibility of Mac mini changes/updates and iLife and iWork updates.
Some have said there is an outside chance that they will be rolling out an updated MacBook or an “ultraportable” addition to the MacBook or MacBook Pro line. We’ve already seen rumors of an ultralight Apple notebook recently, and the MacBook lineup was updated this past May (albeit not much of an update).
There have been some recent patent filings as well as discussions that could certainly be used in a new MacBook or MacBook Pro: AppleInsider mentions a patent concerning an illuminated touchpad as well as improving the enclosures such that thinner and lighter notebooks can be produced.
It’s set to start at 10am Pacific, 1pm Eastern (US)