There is an interesting article in CSCW2011 this year about the performance of different technologies for communicating with servers from within a web browser:
Carl A. Gutwin, Michael Lippold, and T. C. Nicholas Graham. 2011. Real-time groupware in the browser: testing the performance of web-based networking. In Proceedings of the ACM 2011 conference on Computer supported cooperative work (CSCW ’11). ACM, New York, NY, USA, 167-176.
this is available from ACM Digital Library here, or from various other sources (use Google!). Various “hacks” have been proposed in the past to fetch data from servers (long-polling, Flash applets). The article discusses these and others. For me the most important point was the performance that they managed to achieve with Web Sockets in HTML5.