Are developers who work on open source projects keeping their skills more up to date than other developers? That's the case being made by Forrester recently to companies trying to get advice on how to made development a core competency.
It's true that many open source developers are very plugged in. The level of engagement and passion we've seen in working with users of the Coverity Scan project shows that there is a very real community. Anyone who is passionate and skilled enough to make a significant contribution to a high quality open source project should be on the short list of people for any organization to hire.
That said, there are also many open source projects on life support, or nearly abandoned. Part of what makes an open source project valuable is the community behind it; not all project can attract such a community. Frankly, some open source projects are more of an act of vanity, or a sign that the developers don't know how to work well with others (see this still-valuable blog from 2009 on Competition and Open Source Projects).
Like many things in software development, the details matter.