I think this is generally a good idea, but the danger is that programs start checking for implementation instead of supported features (or quirks). It's important to make sure that the dictionary definition stays updated with features (or quirks) to prevent this.<p>See the HTTP User-Agent mess to see what can result if this goes the wrong way.
I think this is a terrible idea. Implementers of Python should code to a unified Python standard, instead of asking programmers of Python to code to a disparate collection of implementations.
I suspect this would be most useful in working around 2.6/2.7/3.1 differences and loading different files/polyfills in response, or something. But old versions arn't going to magically retroactively implement a new PEP, so we won't see the benefit for some time. Other than that it's hard to imagine a use case for needing this sort of information.