The National Review's Daniel Foster has an ordered list of what he considers to be the four most likely Supreme Court nominees, sorted "from left to lefter". In that order, he lists Merrick Garland, Elena Kagan, Diane Wood, and Pamela Karlan.
Most other people that I've read today seem to agree with that ordering -- although many of them seem to consider Karlan less likely and not a part of the short-short list. I don't know whether their characterizations of each candidate's judicial philosophy is accurate or inaccurate -- but it does seem to be the conventional wisdom.
Certainly, there are a few legal scholars and Supreme Court correspondents out there who are qualified to know in something approaching an absolute sense each of these people might preside. For the rest of us, however -- and by "us", I mean other people who cover politics for a living -- we're really just kind of winging it and our impressions of each candidate's ideology will be formulated only in a relative sense.
Say that Obama wants to nominate Kagan, for instance. It would seem to do him a lot of good if the Beltway's impression of the short list were...
...as opposed to:
In other words, I think we can expect to see some trial balloons for relatively liberal candidates like Karlan and Harold Koh -- unless perhaps Obama is planning to nominate someone relatively conservative and might be more concerned about framing the choice to his base, in which case he might dangle some more conservative names instead. Just think of the fun that would ensue on liberal blogs if a "White House official" tells the Washington Post that Obama is considering Ted Olsen! The Overton window is a somewhat overused concept, but it would seem to apply almost perfectly here.