I had a similar idea a while back, here:https://bitcointalk.org/index.php?topic=29102.msg366672#msg366672
Here is what I ended up with, printed in stainless steel via shapeways:
It's simply a bitcoin address as a QR code on the back, with the same address encoded around the rim in a raised barcode. The initial idea was that you could either scan it to send payment (i filled the negative space in the QR code with paint, then polished it nice and smooth), or you could "sign" a document with the rim by using a stamp pad, rolling the rim in ink, then rolling the rim on paper.
3d Metal Printing leaves a little to be desired when it comes to resolution. The QR code works fine, but the raised barcode doesn't. I also printed it in high-detail clear here:
The detail is much much better, but it tends to be a bit fragile on the raised barcode.