I like the look of the Tenori-on. The device appeared in June 2005. Although at the expected launch price of £500 I doubt I would be standing in a line to buy one when it is finally launched. There's a very good article all about it on Create Digital Music which gives lots and lots of information about the device and specs.
It seems as though the unit is going to ship without the ability to network with other devices, which, as far as I understood it was a key element in the design. Anyhow, what I was thinking, was wouldn't in be interesting if an application emulating this was developed for the Palm OS or for Pocket PC for that matter.
The basic design and interface could easily be mimicked. PDA's already have touch screens so that's not a problem. Several application developers have used sampling in their applications and MIDI (albeit for older palms) as well. Also, many newer PDAs have bluetooth and Wifi so networking could be incorporated perhaps.
I think something like this would be a great addition to the handheld music library. I wonder if anyone will take it on?