XMLHttpRequest - W3C Working Draft 20 August 2009
http://www.w3.org/TR/2009/WD-XMLHttpRequest-20090820#the-setrequestheader-method
This is very annoying for Palm webOS, where WebKit is basically the top-level OS and not a browser. Might be a problem for ChromeOS too? Apps can't set their own User-Agent headers: "For security reasons, these steps should be terminated if header is an ASCII case-insensitive match for one of the following headers: ... User-Agent"