This is really big news. Oprah has never endorsed a candidate before. As we all know if she proclaimed tic tacs as her favorite candy everyone would rush out and buy them. If she sneezed and used a hanky instead of kleenex droves of people would swap over. If she says we shouldn't bomb people when they ain't done nuthing to us, then some people who may not have listened previously may pay attention.
Many people are favoring Clinton in the democratic campaign but if you just think of it from the perspective of a media tycoon who has much to earn from campaign races it's easy to see why we should all vote for Obama. (If for no other reason.) Clinton is ripe material, full of flashy, sensationalist stories that can be twisted and turned. The media have already managed to associate her campaign with fear, to mingle it with scandal, and she has managed herself to be unclear about her stance on the war in Iraq and to be caught planting questions in the crowd.
It would be so lucrative and easy to smear Clinton when the campaign race heated up. They could disable her bit-by-bit, picking her apart, until she appeared a hagrid, hairy, lunatic dyke bitch in the minds of most Americans. This is a bit harder task with Obama. He's already bared all in his books and has not yet had the chance to amass a scandalous history and vituperous hate that can motivate to action and bond republicans together as nothing else.