There is currently a discussion how and if a browser should indicate the presence of installed id selectors. I am against "polluting" the user-agent string. Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; WOW64; SLCC1; .NET CLR 2.0.50727; .NET CLR 3.0.04506) This indicates only that .NET3 is installed, it does not tell the relying party whether CardSpace is active or disabled. I prefer that the id selector adds a HTTP header e.g. "X-ID-Selector" to the HTTP requests. This is easy to implement;