[JWait]

Well, since one page is cart.php?mode=checkout and the other is register.php, maybe you can use an "if" statement like

{if mode ne "checkout"}
<input type="checkbox" name="subscription[{$listid}]" {if $subscription[$listid] ne ""}{/if}checked />
{/if}

I'm not sure of the syntax (mode might be $mode, or something else), and again used the newsletter subscription, but that might work. The problem is, it looks like you are using a 3rd party mod, an I don't have the template to look at.