I am leaving for WDW in about 2 1/2 weeks, and just booked a Le Cellier lunch online today. No, I'm not one of those people who has been trying for weeks - I just thought I'd see if something was available, and it was. I'm sure I lucked into a cancellation, but it does go to show you that it's possible to get something even if you're within a few weeks of your trip.
The thing I like the most about the online booking is that if there isn't something available (and even if there is) they show you alternates that are available - sometimes places you might not have thought of. It's so much easier than asking a CM to check here and there until they find something. It is also a big help if you're on property and you just want to see what's available for that day or the next.