What you want is an RFID keyfob/detector. There used to be a writeup on a homebrew version of this at http://hackaday.com/2013/09/07/hidden-rfid-reader-locks-work... -- but the link that article references is rotten now. A sufficiently motivated person should be able to dig up instructions somewhere.
A few years ago there were companies that did this for most operating systems, and fairly inexpensively too, but I'm having trouble finding them now.
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?
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.
A few years ago there were companies that did this for most operating systems, and fairly inexpensively too, but I'm having trouble finding them now.