[padelt]

A simple readonly RFID tag - agreed. But think "smartcard with wireless power transmission" like [1]. These should be able to ensure cryptographically secure communication and identification. At least MitM-attacks without physical access to the card should be unfeasible, right?

[1] http://en.wikipedia.org/wiki/Contactless_smart_card#Security

[thaumaturgy]

I don't think perfect security needs to be the goal here. You just need to have your device automatically protect the data on it when you're separated from it -- whether it's the state in suits, or just random laptop theft in San Francisco.

If you're up against an enemy capable of realizing ahead of time that you've RFID-protected your device, and capable of cloning your RFID tag without your knowledge, then you're probably screwed no matter what you do.