perhaps the folks it is not working on has disabled the ability for their browsers to accept cookies. In MSIE 6, the browser alerts the user every time a web site tries to set a cookie and allows them to block it.
Cookies are great, but you can't totally rely on them working 100% of the time. It is too easy for the client to do something to turn them off. The same holds true for sites that depend too much on Javascript, it takes only a click and the functioning ceases to exist.
Jim
