It depends on a couple of things.
1) Were you late on the previous mortgage leading up to or through the short sale?
If you were late, you need to wait until 3 years from the sale date have passed so you would be good staring 9/25/2012. If you were not late on the mortgage you maybe eligible now.
2) What type of loan did you do the short sale on?
If it was an FHA loan and FHA paid a "claim" as result of the short sale, you have to wait until 3 years have passed from when FHA paid the claim.
3) If you have 20% down you maybe eligible for a conforming, Fannie Mae/Freddie Mac mortgage right now.