In response to a request to confirm or deny reports of Microsoft's participation in the National Security Agency's PRISM wiretapping program, the Redmond software giant promptly responded with the following statement:

“We provide customer data only when we receive a legally binding order or subpoena to do so, and never on a voluntary basis. In addition we only ever comply with orders for requests about specific accounts or identifiers. If the government has a broader voluntary national security program to gather customer data we don’t participate in it.

It's not quite as blunt as Apple's "never heard of PRISM" denial, and you are free to parse those words for yourself, but that sure does sound like a pretty clear denial to me.