exhibit passes (tickets, whatever word you want to use) are encouraged to buy before hand. the big expo in january; macworld, if not already, would be selling tickets. keynote tickets are of limited supply.
these one-off events, like the one a few weeks ago for the new ipod touch, was invite only to press and a few in the tech industry.
macworld 2008 pricing is out; notice what you have to spend to get into the keynote:
Packages & Pricing | Macworld Conference & Expo
exhibit only is NOT going to get you into the keynote.