The second big announcement at the opening day of the D: All Things Digital conference (following Microsoft’s unvieling of Surface, its gesture-based UI) is a new mobile computing platform called Livescribe. Taking the shape of an ordinary ball-point pen, the device can digitize notes written on special miniature-dotted paper, as well as record and playback audio. Then comes the clever bit — double tap on a word you’ve written and the smart-pen will replay a portion of audio at the exact point the notes were taken, so for example, a few keyword notes can be tied to a place in a recorded lecture or interview. Later on the synced data can then be downloaded to a PC where it can be “replayed, saved, searched or sent”.

Livescribe is more than a single application and is intended to be a complete development platform, allowing others to build new software and functionality for the device. Examples on the company’s website include enabling the pen to publish text straight to a blog, conduct arithmetic, and do on-the-fly translations.

Livescribe looks like a very impressive product which is introducing a new category of mobile device — just don’t call it pen-based computing says Livescribe’s founder, Jim Marggraff. Instead he calls it paper-based computing, perhaps wishing to distinguish the product from the limited success of other pen-based devices such as Palm’s line of organisers or the Tablet PC.

Livescribe’s smart-pen will cost $200 and be available sometime in Q4.