The current rumor is that RG3 will stay at Baylor to finish his Master's Degree. Where have you heard otherwise that its "all but certain" that he will declare for the NFL? Not doubting you, just curious.
The rumor of him staying is coming from the Baylor homer who writes for their sport network online. Each university has a writer through rivals.com and some other service (can't recall the name now) and most are students or alums.
RG3 has taken the intitial steps toward entering the draft. Sure, he could have a last minute change of heart. So could Luck. Landry Jones too. But indications see it otherwise.