depends on when you're going & how flexible you are. In both May & September 2008 (less busy times) we saw people getting walk-in seating at most of the restaurants we ate at. But not at the most popular ones (like Cali Grille & Le Cellier.) There were times, though, when the sign on Main Street/MK said there was no seating available at such & such, but when we were there for our ADR they actually were taking walk-ins.
If you're willing to eat early or late, and willing to try less popular places, you'll have better luck with no reservations. I will say for our short vacay in January 20-22, I had no problems getting multiple time choices for our restaurant reservations, just last week. We got 2 breakfasts, 2 lunches, 2 dinners...all table services. That included some popular choices like Chef Mickey & the Coral Reef. So you may have a great time winging it.