I absolutely hadn't but that looks damn interesting, thanks!
What was the delay like in actual usage?
EDIT: This is literally what I was describing and have wanted for ages, US only but some ebay.co.uk vendors have them! I'm having one of these soon!
http://www.myboogieboard.com/na/products/boogie-board-sync-9...
I tried it. I really wanted to like it, but the writing screen was just not bright enough for me. May be it will work for you.