Especially for the iPhone
                +4 
                A: 
                
                
              
            you need to try to set a cookie, then do force a reload via window.location and then read the cookie. if it exists, the browser supports it.
                  dusoft
                   2010-03-24 22:45:25
                
              
                +3 
                A: 
                
                
              This works in IE, Chrome and Safari (which should be the same as iPhone):
if (navigator.cookieEnabled)
   alert("ON");
else
   alert("OFF");
EDIT: Since nvl decided to take my answer and not check into it I thought I should. Tested it on all the browsers I could find and seems to work just fine.
                  Kelsey
                   2010-03-24 22:47:26
                
              I relied on you! :P and I think you are more worried with my taking your answer (i was trying to make my answer cross-browser compliant, i checked only for Chrome and FF) than actually checking, so I delete it.
                  N 1.1
                   2010-03-24 23:32:26
                `cookieEnabled` is really unreliable. See eg. http://stackoverflow.com/questions/2167310/how-to-show-a-message-only-if-cookies-are-disabled-in-browser/2167462#2167462
                  bobince
                   2010-03-24 23:36:34
                +1 @bobince good post. Upvoted your thread :)
                  Kelsey
                   2010-03-24 23:48:30