Antarctica

I have heard about it but can't confirm it for Antarctica.
When we were in Galapagos there was a couple in our boat that was doing it They said that they had several options to pick from. If you decide to try your luck you may want to make alternate plans, the national park neat Ushaia is very scenic. If you succeed, please post your experience here.

There is a company that discounts expedition-type cruising, including Antarctica. You can check the site and/or sign up for a weekly newsletter with the latest offers:
www.expeditiontrips.com
I've seen discounts up to 50% two months before sailing.

I agree with Skunkman that you are taking a huge chance by trying for last minute passage. There is a huge difference between traveling on a huge vessel and a smaller one limited to 100 people due to the IATOA regulations and the Antarctic treaty. And I suspect, if at all, you might find a berth on a huge vessel.
From my experience in 2008, the true essence of Antarctica is being able to spend time ashore -- not just seeing the scenery from afar. You want to find an opportunity that permits you many landings -- we had ten -- permitting you to hike or just sit watching the penguins and other wildlife. You will want someone to help you understand the remains of the seal oil industry or old research stations or the effects of climate on the ecosystem. We happened to go with OAT in 2008 at the end of season in February and were thrilled with the experience.
If you run the expensive risk of seeking a last minute berth, be sure to find a smaller vessel which will enable you to see the essence of Antarctica at your leisure, close and personal. Good luck. I wouldn't exchange my experience in Antarctica for anything. It's an amazing place!

We went to Antarctica in 2007 on a 270 passenger ship run by Hurtigruten (Norwegian Coastal Voyages). It cost much, much less than taking one of the 100 passenger ships. While it is true that a ship can only have 100 people on the shore at any one time, a ship can rotate passengers so that everybody gets to visit each landing for as long as they like. We had 9 landings. We also had several highly qualified naturalists on board and they gave lectures every day. I doubt that my experience was any less than someone on one of the smaller ships. Even though this ship could physically handle up to 400 people, they must restrict themselves to 270 for Antarctica. It was "sold out" at least 2 months in advance, according to a passenger we met who had wanted his brother to come and share his accomodation.

I've been watching the expeditiontrips.com website and they discount the following ships sailing to Antarctica - as for any travel sale, discounts are usually offered about two months out:
Lindblad/National Geographic (capacity 110)
Prince Albert II (capacity 125)
Orion (capacity 100)
Corinthian (114)

We had 9 landings. We also had several highly qualified naturalists on board and they gave lectures every day. I doubt that my experience was any less than someone on one of the smaller ships. Even though this ship could physically handle up to 400 people, they must restrict themselves to 270 for Antarctica. It was "sold out" at least 2 months in advance, according to a passenger we met who had wanted his brother to come and share his accomodation.