Apparently judicial activism is back on the conservative radar with the California Supreme Court decision on gay marriage.
Judicial activism is one of the great myths of conservatives. Not only does it critique courts for exercising essentially any form of constitutional review (See, e.g., Marbury v. Madison), but it is based in a false "separation of powers" argument.
The classic version of separation of powers that we all learned in high school goes something like, "Congress makes laws, the President enforces the laws, and the Courts tell us what the law is."
Never mind that even this simplistic version gives courts great powers, but what is more important is that the divisions of legislative, executive, and judicial power are not in discrete categories. The President, e.g., has legislative powers in the form of the veto. Congress has judicial powers in that it control the jurisdiction of federal courts. The Courts have legislative power in that they can review and interpret laws.
No one claims that the Executive is "activist" when he veto's a bill, or otherwise asserts the power of his office. No one claims Congress is "activist" when it restricts the jurisdiction of federal courts to hear habeas corpus claims. So why is a Court "activist" when it says that a law violates the right of people?
Dahlia Lithwick has a great piece on this here.