Is open source sustainable?

There is an interesting post over on Alex Kings site – he is a plugin developer for WordPress.  The conversation makes me wonder about other open source initiatives like OpenNTF.  I know many people across the community contribute amazing material to OpenNTF and I have often wondered if it was maintainable.  I know that every thing I contribute either in an article, blog post, or on OpenNTF will have some level of “support” in the future.  It may be days, weeks, months or even years but an email, a question, or a enhancement is inevitable.   I would have to say if you are not an open source enthusiast and simply do it because you love it or you in fact do “get paid” enough through donations then your interest over time supporting free enhancements and upgrades will diminish.  I do think many people, especially in the Eclipse and Lotus communities contribute to open source for many other reasons – building a community, promoting a technology, or just polishing skill sets.  Many, like myself, have a vested interest in seeing the community or technology grow so it creates more opportunities in the future – that to me is the essence of open source.