I think it would be cool if it was possible to make a shield with an RFID reader and a solid-state relay switch.  Even if the antenna had to connect by wire or something, it would still be really handy for making all sorts of RFID doorlocks, safes, or totally random projects which need an rfid chip to work.

thinking a mini version of this:
http://www.robotshop.com/productinfo.aspx?pc=RB-See-36&lang=en-US
combined with this (but less beefy):
http://www.cooking-hacks.com/index.php/solid-state-relay-8a.html

Gets my vote