Martin,
Actually this is already possible, although the functionality isn't exposed, it will be soon.
If you don't mind a little hackery, to "fork" a worksheet (the term I use internally for a page) look at the source code of the worksheet you want to fork.
In the body tag you'll see something like this:
<body data-worksheet-id="gI9P1iZ" data-worksheet-ro-id="RU000nE7" data-variables="{}">
If you copy the data-worksheet-ro-id,
RU000nE7 in this case, and create a URL from it like
http://lastcalc.com/RU000nE7, anyone that visits that URL will get a copy of that worksheet that they can edit without affecting the original. I know it's messy, I'm working on it :-)
The reason this kind of thing isn't properly exposed in the UI yet is that I need to figure out how worksheets can be shared, and "published" (allowing other people to "import" functions you've defined). This will require user accounts and I haven't begun that infrastructure yet. When worksheets are published, there also needs to be some kind of peer-review.
So much to do, but I'm making rapid progress. Thanks for your feedback, this is invaluable, please keep it coming :-)
Ian.