depends on what time of year you're there, and what time you're thinking about walking up. We've been there when people were getting walk-ups, but most of the time there's so many people waiting you think there's a convention going on in the waiting area or something.
We were there for an early breakfast ADR (7.30 a.m.) in Jan 2009...the place was mostly empty and they were taking some walk ups. However, 45 minutes later when we left around 8:15 or so, there was 100 people waiting to be seated. And that was during the SLOWEST time of the year (Magic Kingdom, at 10 in the morning...Fantasyland had no lines at any of the rides. NONE. Not even at Dumbo or Peter Pan.)