You want the FAQ (http://us.blizzard.com/support/article.xml?locale=en_US&tag=wowcatatrialfaq&rhtml=true) for that one.
The free trials will not let you exceed your currently purchased level cap.
...any characters you play during the trial can only advance up to one experience point below level 81.
At the end of the trail, any goblin/worgan characters will be rendered inaccessable.
Any new Goblin or Worgen characters you've created will be saved and will be inaccessible until you upgrade to the full version of Cataclysm.
Any characters in Cata zones get ported back to the old world.
Any characters that find themselves still in Hyjal, Vashj'ir, or any other expansion zone at the trial expiration will be transported to their bind point in Azeroth or Outland. If the character's bind point was set in a Cataclysm exclusive zone, he or she will be transported to his or her home city.