IIRC, he was subpoenaed but not called to testify. The judge ruled that asking to get out of testifying was premature since he hadn't been called. (The article I saw didn't say what the subpoena was for -- possibly a request for documents. Maybe for a deposition, but I would think that would count as testifying.)
One of the basic requirements for a perjury conviction is that it must relate to a trial or hearing on a substantive matter, and the perjured statements must be relevant to the case in question. Libby's basic defense is two-fold:
1. The matter is not substantive, since the questions Libby was asked involved a matter so inconsequential that he could easily have forgotten details over time; and
2. The matter is neither substantive nor relevant, since the prosecutor had no business conducting the investigation in the first place (because he wouldn't have been able to file criminal charges even if every accusation against various people in the "leak" case were true).
The testimony of Wilson is important because it ties directly to Point #2. Libby's defense team will probably go to great lengths to point out that the original CIA request for an independent prosecutor was complete bullsh!t -- on the grounds that the CIA couldn't even make a minimal case that any information revealed about Valerie Plame involved a specific violation of a specific Federal statute. This would mean that Fitzgerald's actions in this case are the equivalent of a prosecutor pursuing a murder case even after the alleged "victim" of the murder shows up in court alive and well.