I think that Pres. Obama is a consummate politician. Here's what I think he is saying in his choices of religious leaders taking roles in his inauguration:
Rick Warren: Obama will talk with people he doesn't agree with, he will allow democracy to take place, however ugly it seems. Obama is also open to contrary views, unlike the White House of the past 8 years, where no dissenting thought entered discourse. Also, I think Obama is sending a clear message to his liberal base, y'all aren't getting a free ride, this is a new kind of president.
Joseph Lowery: a bit of the old school, touching the MLK hem, but with Lowery, who is still active in progressive causes, we have socially engaged gospel.
Sharon Watkins: A taste of things to come...I can't think of a better way to put it, this is very exciting...
Right Rev'd Gen Robinson: while pre-inauguration, I think Obama is really just sealing the deal on how is Christianity is big-tent Christianity.
Finally, I find it interesting that all these choices are protestant Christians, no Islamic or Jewish leaders, no Buddhists, Secular Humanists, etc. Clearly, I think, this is a message that says, I'm a Christian, o.k.? Listen here fore more.