The Second, Eighth, and Ninth Circuits have all issued recent rulings striking down the federal partial birth abortion ban, a statute that was passed by Congress three years after the Supreme Court struck down a more or less identical state law in 2000 in the 5-4 decision of Stenberg v. Carhart. The Ninth Circuit's opinion explicitly surmised that Congress intentionally passed the statute in a form that wouldn't survive the Supreme Court's Stenberg holding in an effort to challenge that ruling. This could be the first in a long series of cases directed at reversing recent (and maybe not so recent) 5-4 decisions now that John and Sam are running things.
The 5-4 decision that pops most immediately to mind is last year's Kelo decision, the big fat stinker of a case in which the Court decided that the government can give your house to Home Depot if doing so would generate higher tax revenues. Given the discontent with this case on both sides of the political spectrum I think we can expect Kelo to show up on the chopping block sooner rather than later, to the extent that the Court brings out the chopping block at all (see below).
Another case that people should be worried about is Grutter v. Bolinger, a 5-4 decision upholding a watered-down version of Affirmative Action in public universities. McConnell v. FEC is another one to keep an eye on - the case dealt with campaign finance reform and the First Amendment, an issue that we've been hearing surprisingly little about given the lobbying scandals going down. School prayer and Ten Commandments displays have also been the subject of 5-4 decisions, and the Ten Commandments issue in particular is a good candidate for reconsideration given the way in which the Court fumbled the issue last summer.
An issue that's close to my heart due to a combination of Fed Courts geekdom and Irish anti-royalism is the Eleventh Amendment, which has been muddled by a series of hare-brained 5-4 decisions over the past several years. The Eleventh Amendment has a long and tortured history, and as currently interpreted bars a great deal more suits against state governments that it should. In 1999 the Court handed down the Alden v. Maine decision (written lamentably by one of the five individuals pictured above), holding that the structure (as opposed to the text) of the constitution provides for broad state sovereign immunity, in both state and federal court (the text of the Eleventh Amendment only mentions federal judicial authority). This effectively dislodged sovereign immunity from the Eleventh Amendment and turned it into something along the lines of pee in a pool. I have no idea where Roberts and Alito stand on this issue, though Rehnquist was on Kennedy's side. Let's hope Souter brings them around. Maybe he can sway them with some of his home-made yogurt.
I'm not sure how active the Court will be in revisiting its narrow majorities now that there may be a new narrow majority. If the Court goes buck-wild and reverses a bunch of its own precedents in its first post-Rehnquist/post-O'Connor year, that will demonstrate a profound lack of self-respect for the Court as an independent institution. Let's hope that what we see over the next year or so is a lot of denials of cert and some gritty stare decisis decisions.
UPDATE: This is encouraging, though no reason to break out the party hats just yet.