Tom Warren here (Senior Editor at The Verge). I posted this story at The Verge yesterday. I just wanted to clear up a few things and address some points I've seen posted elsewhere etc. The Xbox 720 leak was covered by The Verge at The Verge, not Polygon at The Verge. I know the temporary home can be confusing at times, but thought it was important to point that out.
As for how we check these types of stories. I have been reporting on Microsoft for around 12 years now. That's not to say I know everything about Microsoft and its processes, but I have a fairly good idea of what is and isn't an internal doc usually after the first few pages. This document in question is from August 2010, prior to iOS 4.2 (mentioned in the PPT notes) and when certain team members (mentioned in doc notes) were still at the company in engineering roles. The document references several employees by name and uses one of Microsoft's internal "CSG_Pres" PowerPoint templates (an early example of their Metro style PowerPoint templates that are used regularly internally now).
Couple this with the fact it aligns with other information I've seen about Nextbox over the past year, it aligned perfectly. The document also references Microsoft's SmartGlass technology (announced at E3). I went through a number of other ways to verify the information was as accurate as other stories we would report on - I'm not going to outline the exact processes because I like to keep those secret :)
We make every attempt to ensure this type of data is accurate. I ran a number of stories ahead of this year's E3, and they were all accurate:
Microsoft to bring full Internet Explorer browsing to Xbox 360 with Kinect controls
Exclusive: Kinect Play Fit to offer universal exercise tracking with 'Joule' heart rate monitor
Exclusive: $99 Xbox 360 + Kinect bundle launching next week with two-year subscription
Exclusive: Microsoft to preview 'Woodstock' Xbox music service at E3
Hopefully this clears up any questions over how we vet this type of information. I don't typically report on Xbox or gaming news (its not my core knowledge) but I do enjoy reading Neogaf threads from time to time. You guys have an amazing community here so keep it up :)