Do Supreme Court justices get more liberal on the Court? If so, why?

On the first question, the pro position is here, and the anti position is here. I personally think that the evidence on Justice Stevens is that he clearly turned more liberal on the court, and Souter probably did as well; but in general, the Supreme Court is a pretty conservative body and O'Conner and Kennedy are simply not liberals. They disappoint conservatives because they're not ideologues.

On the second question, there are several possible explanations for why a justice might shift positions, and I've seen each of these explanations being used at various times by commentators (The comments section of the Volokh post above does a good job laying these out):

1. Liberal arguments are better. (Unless you're prepared to deal with those tricky and seductive liberal arguments, you're likely to be taken in. This seems silly to me, but as a liberal-leaning kind of guy, I have to chuckle whenever I hear its variants, especially coming from conservatives. And frankly, given the options laid out below, it might just be the best explanation out there.)

2. The "Georgetown cocktail set" works its magic on the nominee. (This is a variation on the first position, adding an element of social pressure on the judges. Even sillier than the first reason.)

3. Power corrupts, and so the weaker-minded judges will be more likely to throw off the shackles of judicial restraint than will the stout conservatives. (This at least has some plausibility to it - but you cannot convince me that Scalia, Thomas, and Rehnquist are (were) not very activist. Bush v. Gore is exhibit A. These guys do engage in judicial activism when it suits their ideology. The Rehnquist move to overturn a lot of Congressional laws on the basis of "states' rights" was itself a move to insert the court into policy-making; it would have certainly been more "restrained" to refuse to hear constitutional challenges to federal laws.)

My conclusion: I see no reason why people should get reliably more liberal or more conservative once they get on the bench. I don't know why Stevens and Souter did turn more liberal, except that maybe the court gave them a new perspective on issues that they didn't have before. An inexperience nominiee might be confused and make bad decisions if they don't already have an articulated philosophy, but that is different from an ideological shift.