From what I've deduced there is a lot of strong evidence pointing towards Mumia's conviction. I think one of the main points is that his own testimony about what happened at the shooting came over 10 years after the court case that he did not testify at. It would take an awful awful awful lot of cover-up to from Mumia.

That said, give him a retrial and see if the jury reacts differently. It's only fair within the U.S Justice system.