Deposition requirements
There's no limitation on public acknowledgment, generally. However, terms can be agreed to and become binding by that agreement, and can, on occasion be ordered by the court (if it's in the court system).
It wouldn't surprise me if Pettitte's attorney's negotiated an agreement that the depositions be sealed until the date and time of the Congressional hearing -- in order to give him time to get the heck out of town, and bunker down in hiding from the media (or sit with his advisors and figure how to spin it all when he eventually does face the media). If they have not yet been made public, they will be soon, however. They have been made part of the record of yesterday's proceedings.
But as for Pettitte being bound in any way not to talk about it, that I can't imagine. More likely, the reason he's not talking is because he's hiding and doesn't want to answer any probing questions, most particularly starting with whether, after changing his story, whether this time he finally told the whole truth. I believe that's the one question he most doesn't want to answer because he'll have to continue the charade of lies.
I'm sure he doesn't relish the thought of answering follow-up questions about his latest story on when he and Clemens did and did not talk about this stuff, or the specifics of that/those discussion(s). Nor do I think he wants to reveal the specifics of how he plotted to have his father get him hGH.
I think right now, he's probably just like everyone else -- he wants to put his head down and go back to work and not have to answer any more questions.