Thomas, he's often in shirtsleeves -- I do think he did it on purpose to stand out (and did see the pics of Hillary in her pale green pantsuit), but he's doing it a lot, and I think it works.
February 2nd:

Katrina relief, not sure of date:

in Kenya:

February 20th, in L.A.:

no context
Anyway, you get the idea. Basically, he seems to take off his suit jacket quickly in outdoor, warm situations, and whips off his tie and rolls up his sleeves if the occasion isn't too stuffy. And he usually has on a white shirt, sometimes light blue. (Seems be getting more reliably white lately.) Just seems to be emerging as a signature, and I think that's good (that he has a signature, and that this is it -- the connotations are good).