And if freelancer accepted bitcoins, you could point anyone who wanted to make a few BTC to that site with so many ways to make money.
You are assuming that if they accept bitcoins as payment that they then pay out bitcoins to the freelancer/contractor. That would be nice but each side of it is independent -- they could accept Bitcoin payment and then pay out fiat, or hey could accept fiat and then pay out bitcoin as well. I know these sites have two problem areas ..., one being payment card/bank transfer fraud where the payment gets reversed (or claimed that it was not authorized). The other is with abuse where the "gig" was fake -- and was instead a front for money laundering.
I don't know that a Freelancer will want Bitcoin to be used. Payment fraud (reversals/chargebacks) is one reason freelancer/contractors aren't working directly for others. But with Bitcoin payment there's no chargebacks. Thus there's no risk to dealing directly with an unknown party as long as services are pre-paid by the hiring party.
So Freelancer, oDesk, etc. all rather enjoy the current necessity to use an intermediary.
They are certainly aware of Bitcoin. This is from more than two years ago:
Incidentally, here's a talk by Michael Levinson, VP of Product, oDesk, which has a lot of freelancers from developing nations. He describes the challenges to (and importance of) paying those contract workers. Notice that the bitcoin topic just keeps coming up over and over by those in the audience. (Which is interesting, because this video is [from May, 2011].)
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http://vimeo.com/29287295
