It's time for me to get some sleep, so I'll check out oDesk more thoroughly tomorrow. My first thought though is that, like other freelance sites, employers might expect me to work for pennies on the dollar.
If you were doing SEO/SEM writing or something like that, then maybe, but for these kinds of jobs, you can charge a premium if you know what you're doing & deliver value. Many employers are cheap, but fewer are stupid---they do want to get their projects done.