This change is only for T-Mobile. Basically to test out the features of the new BIS (BWC) before possibly doing the same for other carriers. Lots of new things coming out with it as you can see at
http://www.t-mobile.com/bisupgrade
RIM is sending out PIN messages to all T-Mobile customers to inform them of the upgrade as well as send another PIN message to all T-Mobile customers who use the mail connector to get email from their corporate email account. This is because their is also an upgrade to the mail connector.