g1smd - 9:12 pm on Jan 20, 2013 (gmt 0)
You raise a very important point.
For page numbers that do not exist, return the HTTP 404 Not Found header from your script.
Once a request is rewritten and pointed to an internal script, you're beyond the point that htaccess is handling the request and so the PHP or Perl script must return the right headers and the correct content or error mesage.
Failure to do so leads to the site being flagged for "soft 404 errors", and you don't want that.
For other content pages, such as products or posts, you'll want to return 404 Not Found if they don't yet exist. They'll return 200 OK when they do exist. When they no longer exist, they should return 410 Gone, the status delivered by your PHP or Perl script.
404 - The server can't find it, doesn't know why it can't find it, doesn't know if it ever existed, and doesn't know if it ever will exist. Google will check again from time to time to see if the status changes.
410 - The content is Gone and is probably never coming back (Google will still check occasionally just in case it does come back).