My comment is based mainly on the political situation (enmity between Bush and Chavez, friendship between Chavez and Castro), but there are also a variety of judicial problems that could be cited, besides the corruption in general: Chavez' packing of the Supreme Court in seeming violation of his own constitution; the fact that judges for the most part don't have tenure and can be fired by that packed Supreme Court for making politically incorrect decisions; the fact that Posada was already tried twice in Venezuela and acquitted for the airliner bombing. He escaped (bribed a warden) while the case was under appeal by the government, so presumably (I guess) if he goes back the appeal will go forward and be heard by Chavez' Supreme Court. Or, maybe they would attempt to retry him a third time.
All these things makes it even pretty unlikely (in my opinion) that Posada could ever get something resembling a fair trial in Venezuela. More likely it would be a political circus.
http://hrw.org/english/docs/2004/12/14/venezu9864.htm