Great idea. And it's very relevant to sustaining communities, forums, and blogs - a good revenue model and a nice complement to the direct response nature of ad sense.
Widgets built by interesting brands would be useful. And maybe entertaining.
And since users are in control they could choose to align with those that fit their personal brand.
Brian McAndrews came in last month to speak to some people at our company. He's CEO of aQuantive, the largest digital marketing company. I asked him about marketers using widgets to extend their brand and content into community sites and he said he'd not heard of any brands doing it. They really should - and aQuantive has the technical resources to do this for them. I know a bunch of people who work here who'd put a coke zero widget on their blog. I would.
You'd need to figure out the money angle though - you'd want to help incent true fans to align themselves with the brands.