Many of us have been using multiple screen setups for awhile now. To use more than 2 though, you had to add a second card. Eyefinity solves a couple issues.
1. Up to 6 displays on 1 card
2. You should be able to use up to those 6 displays if it's crossfired with another 5870 (no more disabling Xfire to use more than 2 monitors)
I'm currently using a 30" Dell 3008wfp, 2 26" Viewsonic VP2650wbs, and an Acer H233H. I will probably end up keeping this setup for most everything, and getting a 6 screen stand if the bezels get small enough to make gaming worth it for a second pc setup just for games.