This is my RFID Box that I built for CASA. It was quite a fun thing to build and grossed the largest number of hits I've ever had on my blog so far.
I had a lot of fun creating this. I built it at the London Hackspace which I joined not so long ago. It was quite good fun though hard work really. Spent a fair few nights with a previous MiFARE version but settled with the classic ID-12 chip from Sparkfun. Trying to get the damn iphone sockets together was the worst thing. Still, managed it in the end.
Overall, I think a MiFARE version would be more useful to people.