Sadly they seemed to be solely for advertising, not for use. Still, encouraging to see them.
(With the electric assist in modern velotaxis, these things should be everywhere by now.)
I hear through the grapevine (ie. the Korean blogosphere) that these things are set to become more popular here in Seoul with the pedestrianisation and traffic-culling going on around the tourist areas (post on this coming shortly).
Ah, and FYI the price is 1000 won (about 50p or 80 US cents; the price of a subway ticket here) for the first 10 minutes and 1000 won more for each 5 minutes thereafter.