Yes!
Support for Apple Mac Operating Systems has been made available and is fully supported by Microsoft Online Services Sign In for Mac.
As an online Hosted Service (also known as Software as a Service/SaaS) BPOS is technically accessible from any Operating System with an Internet connection and a web browser, but the Sign In application permits a more seamless experience with desktop integration and accessibility.
Also available is the Microsoft Online Services Macintosh Support Service Description form the Microsoft Download Center.