Login page should seamlessly redirect if already logged in

[#1]

OK, it's unlikely an already-logged-in user is going to visit http://community.plus.net/forum/index.php?action=login but if this is the case it should immediately push to the main page.
Something like
if(isset[user_logged_in]) {header "Location: http://community.plus.net/"}

[#2]

Or redirect to the referring page (if on the same domain)?

[#3]

Wishlist item...