Pernille's blog linked to an excellent post over the weekend.
The post writer offers a dose of reality on pay-to-volunteer programs and explains why overseas programs won't take just anyone. Put another way, think of it like this. Could anyone walk into your workplace and do your job? It doesn't change just because it's in another country. Aid and development work takes training too, and not everyone can do it. As for "I'll do anything, even sweep floors," well of course there are plenty of people who are willing to sweep floors, and many of them already live near the office.