I just found out that the Hövding invisible bike helmet has gone into production and is now for sale. I saw a video about this idea a while back and thought it looked pretty interesting. It's quite expensive — £300 — and also it uses electricity, so it's not obviously better than a conventional bike helmet. I'm not a cyclist so I can't evaluate it further than that, but it's certainly interesting.
If you're a cyclist reading this, I'd love to know what you think.