This is an interesting starting place, but I think it only accounts for some of the situations (and then only with some of the housing stock). At the high end (and that's where the action is, bubble-wise) we're talking about the difference between those who can afford the cash-flow AND have a down-payment and those who can "only" afford the cash-flow (i.e., newly upwardly-mobile: higher income than they've had [why else move?], but not enough savings). If you don't have that, you won't get comparable rents. Also I think it's difficult to discern between sellers who are moving out of the area and those who are moving up.
I think the guy from Columbia was more on it as he talked about the supply-demand issue; I think the places that are needlessly-frothy are just setting expectations based on the actually-frothy places. And good luck with that :-)
The problem with talking about a "real estate bubble" is that there isn't any one market, even in a particular area. The stock market bubble happened in one place ... I think the interesting point of the paper is that some places are bubbilicious, and others aren't or are even undervalued.
/jordan