When I saw this post on kathrynivy.com and the corresponding inspiration I immediately set to work figuring out how long it would take to make one.
For a 6' diameter rug (which incidentally costs approximately $1,394.35 on Hay's website) one would have to knit and felt 2,304 1.5" diameter balls. Some more math: if you knit 1 ball a day, it would take over 6 years to finish. To finish in 1 year, you'd have to knit 6.3 (so 7) balls a day every day. And that doesn't take into account connecting the balls and forming the finished rug.
I suppose if you weren't in a hurry for an spectacularly colorful wool rug, you could poke along and finish it when you finish it. I know I have no place to put such an item in my current house, but I have the vision of my future home with a room off limits to dogs and small children and all my favorite things inside. Could you imagine sitting down on this to spin? Sweet.
ETA: So it didn't occur to me until almost 4 hours later that if I were to undertake this particular project, I would, in fact, be literally knitting with balls. I suppose that means I have to do it, huh? :)