Literary agent Kristin Nelson in her blog Pub Rants listed statistics for her agency in 2006 in a post titled A Year in Statistics. Check it out.
As one commenter calculated: "The odds of going from a query letter to being a new client at your agency, 1 in 2600."
However, another commenter countered that the majority of queries received by agents seems to miss the basic concepts of format (such as grammar and proofreading), story concept (such as plotting issues), and understanding of industry standards (such as staying within word-count recommendations). "Those are within anyone's reach. All they take is a professional attitude and a little effort. Get them right, and your odds improve by at least a factor of 10."
Even at 1 in 260, that's not all that encouraging.
I guess this just illustrates how competitive the industry is and that no author should take rejection personally.