+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
+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
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
`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
+1 @bobince good post. Upvoted your thread :)
Kelsey