As one of the largest owners of customer/contact data in the known digital universe, I think this is a smart move by Salesforce.com. In my opinion, I think they're gaining more out of the data than they are from Radian6's actual product offering. Their arduous and unintuitive interface is not something that many have been a fan of.
If Salesforce.com can successfully and accurately start marrying public social data with individual contacts and groups within Salesforce.com's databases for their customers, the sky is the limit for business development, market segmentation, and targeted communication for both B2B and B2C.
Things to think about
It will be interesting to see what happens to Radian6 as a product. Will they trash the presentation layer and start fresh with a new, more thoughtful interface that is fully integrated with their own product suite or just leave it and add to/improve it?
It will also be interesting to see what happens to all the companies that make their cheddar as a social CRM solution. If Salesforce.com can do it right, this infusion with Radian6's data will render some third party solutions useless. Their only hope may be lower pricing.