Lee-
On the other hand, it would break whatever tools already depend on the old URLs, like Magnus's offline reader, etc.
It is doubtful whether any new scheme would stop search engines from indexing the content. The thing is, search engines try increasingly to get snapshots of dynamic web content, so they grab almost everything.
In addition, existing links would stop working. I may be the only one who does such a bizarre thing, but I occasionally pass around direct edit URLs to Wikipedia articles to demonstrate how it works ("take a look at _this article_, and if you find an error, you can _edit it_ right now").
Perhaps we should return an HTTP error code for known search engine agents accessing edit pages? No search engine I know indexes 403 or 404 pages.
Regards,
Erik