In September of 2009, Microsoft and O'Reilly Media announced a strategic alliance where O'Reilly would become the exclusive distributor and co-publisher of Microsoft Press titles.
Today, Microsoft has informally announced that Pearson has now become the official distributor of Microsoft Press products. The agreement becomes effective on April 1, 2014 and includes the distribution of both print and digital products. In addition, a brand new web site is being constructed that will bring a standalone Microsoft Press Storefront to the web. When the site launches, the URL will be: http://microsoftpressstore.com/. If you go there today, nothing exists, not even a placeholder page. According to a Whois search, Microsoft owns the domain but the web site will reside on Pearson servers, according to DNS records.
Though O'Reilly will no longer be the exclusive distributor of Microsoft Press titles, Microsoft Press books will still be available in the Safari Books Online Library, which is still a joint O'Reilly/Pearson cooperation.
Pearson officially announced the change on March 11, 2014, but since very few took notice of the news, it's possible that Microsoft's informal announcement today is to help build better awareness.
There's no indication as to why the change was needed, but some have speculated that O'Reilly's business may be changing soon.
Of note, any Microsoft Press books purchased through O'Reilly will remain in your digital locker. Just because the distributor is changing hands doesn't mean that the books you own will be taken away.