Well I don't know, I wrote a page that displays back all the info in a the users cookies, I use this for testing. It's a server side (ASP) so it uses Response. and Request. to access/manipulate the cookies I went to this page today and the cookie I sent with a client side script shows in that list, with a value.
My thinking after posting was that a client-cide cookie would not be readable by a server side script due to security or privacy concerns, but this doesn't seem to be the case with my cookies.asp reading my client set cookie. As well I just tested the other way a client side script reading a server side cookie and it was read.
I think initialing what I was doing I was doing wrong, thus I thought that there may be a differnec in client set and server set cookies.
In answer to your question cookies that are longer than the session are stored in a txt file on your hard drive, I.E. Cookies that have a an expiry date, cookies without an expiry date expire when you close the browser, so I would assume thay are stored in memory. If you'd like the code to view cookies with client side script let me know I will post it.