what observation will invalidate theory of inflation?

Recently there was a conference at KITP on future of Physics all talks
of which are archived online at
http://online.kitp.ucsb.edu/online/kitp25/
I was going through the astrophysics & Cosmology talks (quite
interesting) and one question which Peebles has asked to
all panelists is " what observation will invlaidate inflation?"
anyhow I would like to pose the same question here :-)
What observation do people think would invalidate inflation?
Melroy