The guys at Icrontic say that if you complete the final Night Elf mission in WC3 on 'hard' you will see a teaser for starcraft 2.

Here are some screenshots they took.

If you can't complete the final mission on hard, but still want to see the trailer, you can use these cheats:
whosyourdaddy - units become invincible, kill enemies with 1 shot.
allyourbasearebelongtous - instant victory
to use the cheats, just type them in the chat (enter).