Penny has been solid for the Red Sox this year (6-4 5.30 ERA), but with a plethora of starting pitchers, Penny doesn't figure much into their long-term plans.
With Clay Buchholz waiting in the wings and the recent progress of Daisuke Matsuzaka, dealing Penny for a grade B prospect might be their best bet. Olney says it's almost certain the Red Sox won't offer Penny arbitration, so dealing him before the July 31 deadline will at least allow them to recoup some of his value.
The Brewers on the other hand have been trying to acquire another starting pitcher for some time, and Penny would be a good fit. He's familiar with the National League, which will help eliminate any short-term league adjustment that some pitchers often have. Additionally, power pitchers like Penny tend to fare better in the pitcher friendly National League.
Prior to speculation about Penny, the Brewers were believed to be interested in Diamondbacks starter Doug Davis, but all indication was the price was too high. However, the Brewers and D-Backs just swung a deal yesterday for second baseman Felipe Lopez, so maybe their working relationship will help them come to a more compromising deal for Davis, but it's unlikely.