We went in September, the only real ones we saw that were "walk-in" were the RFC at AK during lunch and the T-Rex Cafe, also at lunch. We were able to change/"walk in" to reservations at Rose and Crown, but everywhere we went for dinner (50's Prime, San Angel, Coral Reef, and Teppan Edo) all turned awal walk-ins. When we went in November a few years back, I remember the same thing... Walking by angry folks outside places who were complaining to their families about having nowhere to eat.
I would suggest what was suggested above... Assume you won't be able to walk up anywhere for food, especially during prime eating hours, unless you have reservations. Make some now and if you luck into some there, consider it a gift.