Mountain Gear did it and found the following:

Mountain Gear, delivers even better results when it includes reviews of top-rated products by its customers. In a recent test, e-mail containing customer reviews produced 5%-8% higher click-through, 14%-17% higher conversion and 81%-85% higher average order value than e-mail without reviews promoting the same products. [Internet Retailer]

This makes sense of course – they are effective marketing on a website, why wouldn’t they be in marketing emails? If you’re selling retail items which have customer reviews, this is a no-brainer.

In my own experience with consulting, I’ve had good success sending references including testimonials and mini case studies from the references as part of the sales process. (Typically sent along with a proposal).