If you want to understand media narratives -- and why the press has made Barack Obama into a hero (at least, for now) -- all you have to understand is pro-Wrestling.
If you've ever watched the WWE for any length of time, you'll notice that the writers make some wrestlers "face" guys -- good guys -- and other wrestlers are "heels" -- the wrestler term for bad guys. (At least, that's how it used to work. Now days the wrestlers are sometimes more "nuanced," and it's harder to define them as good or evil)
Here's the thing to note: A wrestler's identity can last for a long time, so long as that character is entertaining and good for business. But as soon as a face guy becomes boring (this could take days or years, depending) the writers will make him into a heel.
Media narratives are similar. As long as a narrative is good for business, it's not likely to change. As a result, the media will play up Obama stories that confirm the narrative -- and downplay stories (like Rezko, for example) that undermine the narrative (mixing narratives confuses people and is bad for business).
In fact, Bill Clinton was on to something when he described it as a "fairy tale." Right now, it's good for everyone (except Hillary) to propagate this image of Obama as the "knight in shining armor" archetype. It sells newspapers, provides easy-to-remember talking points for TV spots, etc.
This isn't to say narratives never change. If a new narrative is more interesting than the old narrative, it might change, even before the old narrative is completely played out. Obama's positive image wont last forever. Eventually, the media will make him into a heel (this, of course, won't last forever, either). The question is whether or not he will be in the White House before that happens ...