If you already reached level 70, you'll find that many instances in the Burning Crusade are locked and require a key to get in. I stumbled upon this blog entry by Heather Newman, titled A guide to getting inside 'World of Warcraft' dungeons. It explains how to get the keys to Shattered Halls, all the heroic instances, Shadow Labyrinth, Arcatraz, Karazhan, Serpentshrine Caverns, The Eye, and finally Battle of Mt. Hyjal.
Similar information can be found at World of Raids, Curse Gaming, and of course WoWWiki.
In any case it appears that Blizzard is prepared for people playing hundreds of hours of the level 70 endgame. I just hope that this time around the endgame is more inclusive, and the percentage of players able to get into a raid goes up.