This kind of keyboard is well known in the wearable computer community. The most popular model is the Twiddler and sells for $200 I think.
Check out http://www.handykey.com
On Fri, 2 Jul 1999, Elizabeth Childs wrote:
> I have heard about a keyboard that could be designed that would be one
> step beyond the Dvorak. It would have just one row of keys, and key
> combinations would signify the letters. So keys 1 and 5 together could
> represent "B", keys 3, 7 and 8 could represent @, and so forth. The
> name of this idea is a "chord board".