If I got a PPD shot in august 2011, how long are the results valid?

Depends. The PPD asseses whether you have had a TB infection.If not, and you are infected tomorrow your test will change to positive after a period of time. So the test is only good the day it is done.
PPD. Ppd is a one or two step test for past exposure to tuberculosis. Results are typically good for 1 year, however you only need them if someone requires you to have the test (school, work, etc.) a positive test does not indicate you have an active TB infection, just that you have been exposed to the organism in the past.