There is no external access to the PostTo functionality at this time, not in any of the RESTful API libraries. The only way to do it is client-side, through the OpenSocial library or Actionscript3 library (I believe the AS3 library has been updated with PostTo, at least - I haven't verified it myself).
We're told it's coming along; no ETA. It'll get here when it does. I suspect (and this is just a guess) that the reason for the delay is that PostTo requires UI components not available outside the user-agent libraries. Like I said, though, that's a guess.