We've had amazing luck w/last-minute reservations, particularly during very popular times...
This past March, 9 days ahead of time, my college son asked if we could go to Disney for spring break...I said sure if we can get a villa, but don't hold your breath!
Took 3 days of one phone call per day, but we got five nights in a one-bedroom near the Hospitality House at OKW...the reservation was complete 6 days prior to leaving! It was spring break, one of the three busiest/most expensive times at Disney...
For our early January 2007 trip, we the decided the week before Christmas (about 2 weeks prior to our trip) that we wanted to add 3 days (including a Fri-Sat) to our existing 5-night reservation--turns out that the weekend we wanted was marathon weekend...again, it took 3 days of calling, but we got all 3 nights ... we had to resort-hop the first two nights (OKW on cash one night and SSR on points the other night), but we were able to add the 3rd night to the start of of our BWV reservation...
Finally, last week my non-planning college son decided he could go to Disney the first week of August (a few days less than a month out). We had choices of both studios & one-bedrooms for 8/5-8/11...
So...there are no guarantees and it may be that we were the lucky beneficiaries of last-minute cancellations--but we've had great good luck w/last minute reservations...of course, it helps that we absolutely love both SSR and OKW--they are the resorts that most often open up at the last minute...altho for the January 2007 trip we could have chosen VWL, too...you just never can tell what will open up in those last 30 days, when automatic wait lists stop..
Good luck...