Its actually generally a lot easier to profit buying a bubble than shorting it.There are a lot more points of increase than points of collapse. Paulson made a fortune shorting the housing market,plenty of people predicted the collapse but Paulson picked the peak correctly. The interesting thing is given his shocking trading performance since then he appears to have been lucky rather than smart.

Didier Sornette has some interesting theories(and dubious models) about bubbles,well worth a read.