Artur Janc and Lukasz Olejnik have published a whitepaper outlining CSS history techniques along with results of what they found from real world users. From the whitepaper

"Browser history detection through the Cascading Style Sheets visited pseudoclass has long been known to the academic security community and browser vendors, but has been largely dismissed as an issue of marginal impact. In this paper we present several crucial real-world considerations of CSS-based history detection to assess the feasibility of conducting such attacks in the wild. We analyze Web browser behavior and detectability of content returned via various protocols and HTTP response codes. We develop an algorithm for efficient examination of large link sets and evaluate its performance in modern browsers. Compared to existing methods our approach is up to 6 times faster, and is able to detect as many as 30,000 links per second in recent browsers on modern consumer-grade hardware. We present a web-based system capable of eectively detecting clients' browsing histories and categorizing detected information. We analyze and discuss real-world results obtained from 271,576 Internet users. Our results indicate that at least 76% of Internet users are vulnerable to history detection; for a test of most popular Internet websites we were able to detect, on average, 62 visited locations. We also demonstrate the potential for detecting private data such as zipcodes or search queries typed into online forms. Our results conrm the feasibility of conducting attacks on user privacy using CSS-based history detection and demonstrate that such attacks are realizable with minimal resources."

Whitepaper: http://w2spconf.com/2010/papers/p26.pdf