I think that Microsoft Corp. v. i4i Limited Partnership, which was argued before the Supreme Court on April 18, may change how this plays out in the end.
"Since 1983, the courts have followed a clear, firm rule: In order to overcome the statutory presumption that a patent is valid, a litigant must provide clear and convincing evidence that a patent is invalid. That's a high hurdle to overcome. ... Many observers expect the Supreme Court will reject the current bright-line rule and, at least under some circumstances, make it easier for parties to attack the validity of patents." - http://www.abajournal.com/magazine/article/court_may_make_it...