Salesforce Chatter is a Blackberry BBM competitor which allows users to collaborate using a private and public social network, and allows messages, files and other data to be pushed to users.
Unfortunately Salesforce does not make a Windows Phone 7 version. Dell, which recently dumped all their Blackberries for Windows Phone 7, asked Seesmic of Seesmic Twitter fame to create a Windows phone 7 client for them, which according to the video apparently worked quite well.
Windows Phone 7 is still pretty short on enterprise and business applications, which makes such moves still relatively significant.
The service is built into Seesmicâ€™s twitter client, which can be found here.