g1smd - 12:25 am on Mar 27, 2011 (gmt 0)
I would 301 redirect all those pages to the real site so that customers get to the right page and can start buying.
If you simply return 404 or 410 then you are blocking customers from making a purchase. Indeed, some might conclude the site has closed down.
Once Google no longer lists any of the dev site pages in the SERPs, I would put the dev site behind a .htpasswd access control. That's an action for close to the end of 2011.