There's an article in the latest Velovision about a trike that's hinged in the middle. The rear section can be unclipped and replaced with alternative rear sections. Or, it looks to me like you could unclip the rear section and ride the front as a unicycle.
I haven't checked the site in depth, but I suspect the Shortbike has a 'one-way' transmission, which would make it difficult to ride as a unicycle.
Doh! You're almost certainly right, of course. I was so excited by the apparent unicycling possibilities that it didn't occur to me that it probably wouldn't have a fixed wheel.
