Differentiating pimpl idiom classes in c++

What are good strategies for selecting, either at run-time or compile
time, various pimpl'ed implementations?  Also, retaining the ability
to switch implementations without recompiling.

Boost has an example but with only one implementation class:  (what
would an example with 2 implementation classes look like?)
http://www.boost.org/doc/libs/1_35_0/libs/smart_ptr/sp_techniques.html#pimpl

The pimpl'ed class cpp file has to include at least one implementation
header file.  Is there a method to avoid changing that included header
when switching implementations?

Or, are we using the pimpl idiom incorrectly?

thanks,
graham

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