It's apparently waning among the press corps. With no Clintons to kick around, reporters find Obama less enchanting.
No mystery really. Cozy up to a human being (presuming the label can be accurately applied to a reporter) and he/she'll love you back. John McCain has built a sizable pile of cozying credits with the Washington media. Obama doesn't seem to be the cozy type -- which is fine -- but he did what press cultivation was necessary as a primary challenger. As a front-runner, he apparently hasn't been so accommodating.
I'd be inclined to say "so what?" except that we've learned from bitter experience in 2000 and 2004 that care and feeding of the whiny, fickle press herd can be critically important in the development of decisive electoral themes. The themes are rarely related to substance, of course.