Magic 8-ball say: "Don't count on it".
TiVo Inc. is definitely moving in the direction of being a closed and locked-down system. Given this, I can't imagine them exposing any of the hardware, firmware and software to 3rd party developers. And without 3rd party developers, a TiVo app store would be mighty bare.
As a member of the elite
TiVo Advisors Panel, my best guess is that TiVo will retain total control of what appears on its menu, and will continue to base the decisions as to what changes they make to the menu based on what lucrative B2B partnerships they make on their own, by using focus groups, and automated market research, in that order of priority.
But it never hurts to dream...
EDIT: Remember that TiVo changes SDKs nearly as fast as politicians change their stories. Don't read too much into yet another SDK, especially since it relies completely on Adobe's proprietary IP. I don't see any announcements saying that Adobe and
TiVo are proud to announce some kind of contract that would be needed for any real work to get done.