The idea of a common virtual machine is used for .NET, and, more popular here, parrot ;-).

The big difference is that .NET is optimized for statically typed languages, and parrot for "dynamic" languages.

The parrot developers are working hard on implementing all kinds of languages, and once they have released version 1.0 I'm sure they will gain much more interest. Especially if they manage to make it very fast. I believe they will ;-)

That will make sharing of libraries much easier, so you'll be able to code in "Perl 6 on Rails" if you want, or Python with Catalyst - fill in your favorite language and library.

The big problem, as always, is to convince enough people to focus on one platform (be it a VM or something else).