The transponder is a bad idea. I think nowadays folks are getting uncomfortable showing their faces, much less broadcasting anything else nowadays, especially with all of the monitoring going around.
A modern cell phone is effectively a transponder, we just don't realize it as such, plus the information is publicly available to where stores et al are able to track your visits just walking through the door.
Then there was that movie (name escapes me) where the lasers were scanning eyes and reading retinas and giving directed advertising as you walked through the mall. I'm a fuddy duddy, but no thanks.
I like the idea of a piece of jewelry, like the ring, because it uses a social contact as a form of formal contact.
I'm more comfortable with bumping handhelds. Touch your phone to my phone, perhaps with a quick gesture. Sure there's some concept of incompatibility among devices, but consider today. We have ubiquity of communication (just about everything and anything is transferred over HTTP nowadays, "the Web"), but a disparate range of formats. However with tagged data formats, especially for something as simple as demographics, systems can readily "intuit" much of the information implicitly. Modern cell phones do very well with info in emails and web pages, and that information is not marked up. When it IS marked up, there "intuition" has an even better chance of being successful.
But even if "pulling out the first name" is not 100% accurate each time, the information is STILL captured completely. Then it's a simple task of the user clarifying the machines choices. "No, that's not the first name, this is". From there the machine can readily "learn" from this correction, and next time it sees the same format, it will be able to parse it properly.
Even if the information is captured in just raw text, there's still value.
The real benefit over the high tech solution of a classic business card is simply that it's disposable, and readily left behind. "Oh, Mr. Smith isn't in. Please give him my card and have him contact me at his leisure." Hard to do with a handheld or any other "have to be there" piece of jewelry or equipment.
And, again, with a modern handheld, much can be achieved by simply pointed the camera and scanning the card, even without some formal data format like a 2D Matrix barcode.
The exchange needs to be simple and casual. At the moment, frankly, I think it's awkward having two people staring at their handhelds keying in information. It's not personable as you're staring at the device, and not the contact. A business card or a handheld bump is quick, and casual, and both effect information transfer while not distracting from the actual person to person contact.
This is why you want your cards or handheld as handy as possible, rather than digging through your wallet. You have to take your eyes away from the client, and it breaks the mood. A casual dip in to a pocket to pull out a card, and there is no disruption.