This is true and happened with 1.5 and with the Nexus One: new version dev in private, then a big open sourcing effort (by specific team members) after the version is released. I guess the difference here is that for 3.0 they indicated that they made a lot of tablet-specific changes which won't translate well to phones. So the source isn't in a state where they want to release it for phones, and get the opposite of last year's fiasco (when OEMs put 2.x onto tablets).