Microsoft had a 'priority email inbox' way before Google, and has patents to prove it
Not only is there a slew of research papers detailing how the technology works (it sounds very similar to Gmail's priority inbox), but Microsoft also has a bunch of well-targeted patents filed as far back as 1999!
Is this just another case of Microsoft being beaten to the market? Back in 2007 Microsoft Research released the Outlook Mobile Manager which seems to forward prioritized email to your mobile device. TechFlash also points out that some of the announced updates for Hotmail sound like they use the same Microsoft Research technology. I think this is more a case of Gmail simply being more nimble than Hotmail; a case of Google being less bogged down by bureaucracy and conservatism. I fully expect Hotmail will receive the same (or better) priority inbox technology in the next few months.
And once Hotmail has a priority inbox, I can't imagine Microsoft will continue to let Gmail use the same technology -- especially if Google offers the priority inbox to its business and enterprise Google Apps customers. But at the same time, I can't believe Google would spend time developing and implementing a service only to have it quashed by Microsoft. I wonder how this one will play out...