I accidentally discovered that it is possible to see a competitors CPC through a bid management tool used by one of the larger Dutch SEM firms (the tool probably has a lot more international users).
BidBuddy, an Adwords bid management tool created by The Search Works leaks valuable Adwords data, such as CPC through its cookies. The bid management tool redirects visitors through the domain tsw0.com where it sets a cookie with the Adwords data.
The CPCs etc. can easily be discovered by using Firefox with the LiveHTTPHeaders plugin. If you open the LiveHTTPheaders screen before you click on one of the sponsored results you can literraly just read the CPCs of your screen. Just take a look at the following (Dutch) examples to see how it works.
The Search Works probably has a lot more (inter)national customers who use Bidbuddy to manage their Adwords bids (given the hits on this article…). I would advice anyone who uses this bid management tool to contact The Search Works for advice.