Bombardier designed a concept bike called the Embrio. Not sure if it will ever go into actual production, but it looks awesome! I like motorcycles but I'd like to try this out as well. How can a one wheel (actually, there are two, but the second one only comes down when necessary) possibly stand up against gravity? I suppose the same way that a person on a unicycle can withstand gravity. More on this as soon as I have done more research :)
For now, here's an image of the Embrio:
04.16.10: So I'm back again with a little bit more research under my belt. Since I don't want to re-hash old news, I'll leave you to read an article on the Embrio at Wired.com. Here's a new term for you: vaporware. This is basically an idea that's only ever made it on paper and has already passed its purported public launch date. Sigh...the Embrio is basically vaporware.
Which isn't to say that we can't discover the wonderful world of gyroscopic physics :P