This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
One thing that I will tell you, so that you can look for this behavior in your tests, the browser will limit the number of simultaneous requests made to a particular server. This includes requests for things like images as well as Ajax requests.