I'll have to admit I'm a bit surprised the Cataclysm free trial is already available, what with the massive first day sales numbers, but so it is. If you haven't decided yet whether you want to buy Cataclysm, now is your chance to play it for free for 10 days.
It's important to note that this is not a trial for new players, as you must have an account with World of Warcraft, Burning Crusade and Wrath of the Lich King all activated. And though the update from Blizzard states you'll need a level 78 character as of this writing, you'll actually need a level 80 character to experience the Cataclysm content.
This trial is pretty much intended for those of you on the fence about the expansion, that were put off by the content in Wrath of the Lich King. If you haven't gotten to 80 yet, you can wait a little longer before enabling it. No point in wasting the 10 days, right?