Hey, I've been doing some gigs in oDesk and when you're working on an hourly billed project it's quite safe. They have a piece of software you install that randomly takes print screens while you're working, and makes sure you're active by measuring keystrokes and mouse clicks. You also do daily reports and so the guy who is paying you basically knows everything that is going on. If things eventually don't check out they can appeal on the payments based on the activity and end result, but I have no experience with that.
To me that environment sounds awful. If getting up to think hard about what I'm developing would tend to get me dismissed from an oDesk project, they're implicitly selecting for willingness to just sit and type relentlessly even when that's not in the best interest of the project. It might be appropriate for data entry, but creative work with complex requirements is not well served by a WPM metric.
You are not paid based on these metrics, they are there so that the employer can have an idea of what's happening. I'm sure everyone will be very pleased when you deliver a project update on time even if you record 0 key strokes.