The Daily Howler cites this NYT op-ed as a possible well-founded definition of judicial activism. While the study is interesting (and Somerby is right to say that such studies are more useful than our normal discourse), the particular definition doesn't attempt to correct for political bias. If Thomas has voted to overturn more laws than Ginsberg, perhaps that merely means that Ginsberg agrees with more current law than Thomas, not that she shows more deference to Congress. If the bulk of current law were passed by legislatures more liberal than the conservatives on the court, we might expect less deference on purely political grounds.
While I doubt an attempt to rate individual laws as conservative or liberal would be productive, it might be interesting to look at the legislatures that passed the laws and/or the executives that signed them. Do the justice's records change when laws passed by the party that nominated them are challenged?