My bet for macworld, is the MacBook Air, an ultrathin, ultralight notebook, running leopard. Why?
3 reasons:
1: There has been a lot of speculation, as to Mac introducing a sub notebook, or ultrathin, or ultralight, or even entirely wireless. The MacBook Air seems an appropriate name.

2 : Apple have put a teaser banner on to their website, and then banners at the expo, saying “There’s Something in the Air, Watch the macworld keynote, Tuesday Afternoon, On Apple.Com

3: Just to do a quick url check, macbookair.biz, macbookair.us, macbookair.comp sites are all registered, BUT,
********************************THIS IS THE BIG NEWS*****************************
yesterday, at GMT 2100 to 2300, http://www.macbookair.com forwarded its self to the apple website homepage, saying “www.apple.com/Will+we+see+a+macbook+air+at+macworld+tommorrow+”
in the address bar.

THIS COULD BE ONE OF TWO THINGS: Apple being very cryptic, but telling the truth about the keynote today (gmt) and, hoping nobody would notice (BUT I DID!!!) OR…
Apple are going to reveal something (in the air) that’s completely different at Macworld.

ONLY TIME WILL TELL, (10 HOURS, SINCE I POSTED THIS COMMENT)