These are Skywalker Tallbikes, 12-foot-tall bikes built by Brad Graham. Jesus Diaz of Gizmodo asked him the obvious question: how in the world do you get on top of one to ride it?
JD: OK, and now the really OMFG part: how the heck do you ride this thing?
BG: The bike is very easy to ride if you trust it. Climbing the ladder while it is in motion takes some time to get used to, but after a few tries, I was able to get to the top with a coffee in one hand. Many people tried SkyWalker, and had no problem at all riding it.
Link (with photo gallery)