So, since Apple isn't at MacWorld this year, this 'event' is basically taking the place of MacWorld for Apple announcements.
What happened at previous MacWorlds?
1. MacBook Pro upgrades (and PowerBook upgrades before that.) So we really should see an Arrandale upgrade. I *REALLY* hope we get a Clarksfield upgrade option (i.e. at least one quad-core option.)
2. iPhone update news, but not actual releases. So we may see an announcement of what is to be in iPhone OS 4.0, or what the Summer-release iPhone 3GSuperPlusPlus will be. (We don't have 'real' 4G out yet, so we won't see an 'iPhone 4G'.)
3. iLife and iWork updates; although these seem to have moved to a two-year release cycle, so we may not see these this year.
4. Often see iMac and/or Mac mini upgrades about this time. We just saw an iMac update, so the only possibility I see here is replacing the Core 2 Duo models with "Clarkdale" Core i3/i5 models. The Mac mini is due for an upgrade to an Arrandale, though.
5. The surprise. iPhone, MacBook Air, Time Capsule, iPod shuffle, Mac mini, and Safari were all 'surprise' announcements in January.
So don't get all worked up about a Tablet. He could devote nearly the whole show to it (as he did with the iPhone,) but I expect that if it happens, it will be the gotcha at the end.
Then again, with all the pre-launch hype, he might snub his nose, and not release it until one or two weeks later...