As you probably heard, the BlackBerry service for Microsoft BPOS-S is now free! You can read about this and other technical stuff on the Hosted BlackBerry Service Onboarding Guide.
Purchasing Hosted BlackBerry Service
Hosted BlackBerry for Microsoft Exchange Online is purchased as an add-on to Exchange Online Standard or the Business Productivity Online Suite (BPOS) Standard from the Microsoft Online Services Customer Portal. Please note that effective March 15, 2011 that new Hosted BlackBerry subscriptions are available as a $0 add-on to your BPOS Standard or Exchange Online Standard subscription.
Overview
Instructions for purchase, order and deployment of the Hosted BlackBerry service for Exchange Online Standard.