I think that you are right in that I should work on a visible Open Source project, even if only contributing in small bugs and documentation. Eventually, and with at least some projects under my belt I hope I can land a small position somewhere.
Yet, I don't feel exploited at all because the money issue was discussed with me in an upfront manner and I have not done any amount of work yet. And besides, it is a position that offers experience that I can put later on my CV and potentially eventually it will lead to a paying job.
So in a sense, isn't it riskier to not accept this offer?
I dunno, I still have some doubts...