When does a regulatory taking become a physical taking?

Slate has a piece today on Jay Bybee's 9th circuit opinion that rent control is per se unconstitutional.  The reporters argue that the opinion is silly--that the courts have long upheld the right of municipalities to regulate land use (Euclid v Ambler being the iconic case that enshrined the rights of communities to zone).  But the Supremes have also maintained that when a regulation deprives owners of property of all economic value, it consitutes a taking under the 5th amendment and thus is either impermissible or must be accompanied by compensation to the property owner (Lucas vs South Carolina Coastal Commission).

It is pretty clear that rent control (something that I dislike based on both equity and efficiency grounds) is consistent with the ability of communities to regulate property for a public purpose.  On the other hand, if rent control were set at zero, then it us pretty clear that it would amount to a regulatory taking.  The question is the level at which rent control would require a taking.  For instance, if cities told landlords that they had to cut their rents by 50 percent, I think one could make a pretty good case that the takings rule would apply.  But this is a legal issue and not an economic one--I am not sure whether one could draw a bright line at which a regulation consistutes a taking.