I think that Blank's model about how bubbles inflate or deflate is fine, but not particularly pertinent to whether we are in a bubble now. Horowitz, by talking about public valuations (and referring to Google, Yahoo! etc. in particular) misses the point entirely.
The bubble, if there is one, is entirely private, and the valuations, if they are generous, as VC valuations, not public market valuations. If young startups are getting funded at valuations that require exits at dramatically higher levels than they historically have, then it's a bubble. And this is almost certainly happening now as VC funds left over from the last boom seek something, anything to invest in hoping they will get the next Facebook. Wealth angels who cashed out in web 1.0 or web 1.5 are also pushing up early valuations.
I don't know if the bubble will spill over to public markets or if it will cause silent destruction amongst VCs and institutional investors. The well known private companies with strong earnings (Facebook, Zynga) will probably do fine, but I don't know how Foursquare, no longer hip, will ever justify its $500M valuation. Even twitter will lose its luster if it stops growing (which I believe it has). People think Groupon sucks and it isn't even public yet.
After these guys the bench lacks glamour, and both Pandora and LinkedIn aren't anything like the Netscape boom that started the party in 1997.
The defining characteristics of bubbles is that some chump is left holding the bag at the end. In this bubble, the chumps are VCs unless they can trigger a big stock market boom.