From PolitiFact:

This attack is false, but it’s more than that – it’s malicious. It unfairly tars not just Obama, but all the other prominent, well-respected Chicagoans who also volunteered their time to the foundation. They came from all walks of life and all political backgrounds, and there’s ample evidence their mission was nothing more than improving ailing public schools in Chicago. Yet in the heat of a political campaign they have been accused of financing radicalism. That’s Pants on Fire wrong.

I highly recommend reading the whole article. It covers the founding of the foundation (by a republican), and how the majority of the partners in the program were completely non-controversial, and dare I say it, conservative. It’s this fear / smear that makes me blanch at the idea of a McCain victory. He’s not leading, he’s tearing down. It’s not what we need now.