It looks like Microsoft just revoked several of the certificates of their intermediate CAs. According to their announcement, they revoked the following 28 certificates:
Microsoft Genuine Windows Phone Public Preview CA01
Microsoft IPTVe CA
Microsoft Online CA001
Microsoft Online Svcs BPOS APAC CA1
Microsoft Online Svcs BPOS APAC CA2
Microsoft Online Svcs BPOS APAC CA3
Microsoft Online Svcs BPOS APAC CA4
Microsoft Online Svcs BPOS APAC CA5
Microsoft Online Svcs BPOS APAC CA6
Microsoft Online Svcs BPOS CA1
Microsoft Online Svcs BPOS CA2
Microsoft Online Svcs BPOS CA2 (2 certificates)
Microsoft Online Svcs BPOS EMEA CA1
Microsoft Online Svcs BPOS EMEA CA2
Microsoft Online Svcs BPOS EMEA CA3
Microsoft Online Svcs BPOS EMEA CA4
Microsoft Online Svcs BPOS EMEA CA5
Microsoft Online Svcs BPOS EMEA CA6
Microsoft Online Svcs CA1 (2 certificates)
Microsoft Online Svcs CA3 (2 certificates)
Microsoft Online Svcs CA4 (2 certificates)
Microsoft Online Svcs CA5 (2 certificates)
Microsoft Online Svcs CA6
I'm sure that there's a good reason for Microsoft doing this. And I'm fairly sure that we'll learn the reason thet these particular certificates were revoked. Until then we can just speculate.