I think we all ought to be very excited about MW2008; I'm sure a teaser on apple.com on January 1st will get the laggard Apple fans properly pumped for the weeks following. It's my personal feeling that a solid number of Mac hardware will see updates at MW2008 or very shortly thereafter. I think there will also be some significant software updates as well.
Some of my speculation is clearly informed by reading rumor sites.
On the software front, I think MW2008 and the weeks immediately following will bring:
-10.5.2, which despite being only the second update to Leopard, will be more than a simple maintenance/security release IMHO
-The most substantial iPhone software update yet, with support for the SDK, a new Apple app, and enhanced functionality like video recording
-iTunes 8, with movie rentals costing $1.99 for three days' use, and support for the eventual storefront for iPhone/iPod touch developer apps.
-An Apple TV software update that will mainly bring iTunes WiFi to the device. I think we'll get a nice price drop, too, by the way.
On the hardware front: Upgrades to the Pro lines. What, did you think MacWorld was a consumer-focused show? Well, before last year, we thought it was a Mac-focused one.

-New Mac Pros. Believe.
-Updated MacBook Pros with a case sprucing up
-iMacs and MacBooks untouched for the time being
-3G iPhone in May (probably not mentioned in keynote)
-I'm still iffy about the Mac Mini's fate.
-And the star of the keynote: TouchBook Pro. First off, I think this will be designated as a "Pro" machine, with a base TouchBook arriving later in the year a la the MacBook Pro-->MacBook. Second, I'm reading arguments for a tablet computer and arguments for a slim notebook. I believe the TouchBook Pro will be a tablet/slim notebook hybrid, combining the best of both worlds. It's clear that Apple's fond of convergence; Jobs made much ado that the iPhone was a mobile phone, "widescreen"/touchscreen iPod, and Internet communications device all in one. In other words, I guess we'll see Apple's slimmest notebook yet, making surprisingly practical use of multi-touch in place of a trackpad (i.e. the 13" display isn't what is multi-touch). Flash memory is a probable inclusion; an optical drive is doubtful. Price? I'm going to guess a start at $1599.