A couple living in a SoHo loft decided to put their daughter in a tent instead of in an actual room. The NY Times reports that it is "portable, unbreakable, and won't fall down," all good criteria for a room.
They used the Eureka! Tetragon 8, which has about 64 sf of space, and lined it with those interlocking foam alphabet tiles. Which is a good thing, since the 9-month old kid sleeps in a regular little bed; if she moves around even half as much as our kid, she must be falling on the floor all the time.
Other than that, I think it's all pretty cool.
Eureka! Tetragon 8 tent, $79.99 at thetentstore.com
60 12x12 foam numbers & letters puzzle mat, $39.99 at Toys R Us/Amazon
But if she's tall and a climber, it's much safer than a crib. A short roll onto the floor from what looks like a futon beats a tumble over the crib rail.