Justice Souter is reportedly retiring. If he does, in fact, retire, this would present President Obama with his first opportunity to fill a Court vacancy. I would also consider this a litmus test of sorts. President Obama is a Democrat, and Congress is majority Democrat with a near-filibuster-proof majority in Senate. In other words, it's going to be rather difficult to not get his selection approved - unless he chooses a Harriet Miers.
The test comes in the form of "what type of Justice does President Obama nominate?" Does he nominate a judge who is far left? Does he nominate one who is moderate? Does he nominate one who is conservative (don't laugh, Souter was a GHW Bush appointee)? I think it's going to be telling as to what President Obama's true political philosophy is.
My hope? That he continues to show his more moderate streak.