I am using the Violet Mir:ror[1] as a reader, although Violet have filed for bankruptcy.

It would also be easy to build your own[2]

[1] violet.net/_mirror-give-powers-to-your-objects.html

[2] tinkerit.myshopify.com/collections/arduino/products/rfid-kit

Yeah I'm using custom software, a modified version of Andrew Godwin's python-mirror[1]. It works with the official Violet mirware too but that's less user friendly in this context.

[1] bitbucket.org/andrewgodwin/python-mirror/

Ooo, thanks for the link. I love my Mir:rors, and I wish Violet all the best, but I'm glad there software out there to get some functionality out of the Mir:rors if Violet doesn't bounce back.
On compatible RFID: of course the Violet stuff is all compatible with the Mir:ror, including the Ztamps that you can put in/on just about anything. I wish I knew what else was compatible, too.

I think any mifare tag works, including the cheaper touchatag stickers.

Yup, any RFID reader would do. I'm using the Mir:ror because the hardware is fantastic with it's audio and light feedback on tapping. I have a touchatag too but that doesn't seem to work with an Oyster card.

This could work with non-Oyster RFID cards too. Hopefully including the IBM ID cards in use at BarCampLondon7.

It's a compliment.

That would be lovely. Something like [1]. Ideally the venue would only have an RFID tag or 2D barcode on the wall and the phone does all the hard work of scanning and checking in.

[1] nearfield.org/2009/04/iphone-rfid-nfc

When it's done, no certain date for it. Would you want to deploy and use it?