I've heard a few additional details about what was supposedly discussed today from my sources.
The new unified store is supposedly going to be part of the next version of Windows, which I believe could be Windows 8.1 with the Spring 2014 GDR update, according to one of my contacts. (Another contact said no timing was shared as to when this will happen.) I'm not clear if Microsoft also demonstrated or outlined a plan to make that store the same one that is featured on Xbox One, some time post launch of that console this November.
I'm also hearing Windows Azure is likely playing a role in this scenario. I'm thinking that this might have to do with cloud compilation of applications. This would involve Microsoft compiling developers' applications for them, like they did with Windows Phone during the move from Windows Phone 7 to Windows Phone 8 to help with portability and performance.
The other Azure possibility: The back-end code for certain apps runs on Windows Azure and is streamed to phones and PCs. That would give users the impression that Halo 4 really is running natively on their Windows Phone or Windows RT tablet.
None of this is too surprising -- beyond the possible timing. Microsoft execs have been hinting, if not outright claiming, that in the future there will be "One Windows" and one Windows API (meaning WinRT, I would assume). At the company's Financial Analyst Meeting last week, Executive Vice President Terry Myerson reiterated that vision, noting "“We should have one set of developer APIs on all of our devices. And all of the apps we bring to end users should be available on all of our devices.”
In addition to making developers happy, a single store that included both Windows 8 Metro-Style and Windows Phone 8 apps would definitely help with the never-ending complaints about shortages of Windows 8 and Windows Phone apps.
I've asked Microsoft if the company has any comment on claims they've shown off the common store today. A spokesperson said the company had no comment.
Update: I erroneously noted Microsoft demonstrated a unified app store during the company meeting today. There was no demo; just more talk (yet again) about the fact this is the goal. I updated this post to reflect those facts on the evening of September 26.