This is exactly the type of mentality we employ when trying to hire folks for the FarmVille team. We tell our recruiters to put job descriptions out for "Software Engineers" versus "Flash Developers" and "PHP Developers".

For us, it's rare to find someone with both PHP and Flash experience. Heck, most of the senior devs have less than 2 years experience using both at the same time. So, instead, we look for really smart programmers with solid software engineering fundamentals since we figure that it's much easier to teach a smart person a new language than try to teach someone core CS concepts.

We've seen this work fairly well with several new hires. The language is, like you said, just a medium and changes depending on the project. The real challenge comes from understanding how to architect code in a way that's flexible/scalable/maintainable.