Peter Qian, a developer on the data services team, has posted a nice write up describing the row count feature in the ADO.NET Data Services v1.5 CTP1 release we made available for download yesterday.
You can check out Peter's Row Count write up here.
Also, we have created a new forum dedicated to providing feedback on "pre-release" versions of data services such as this CTP. Please direct all questions and comments about this feature to this new forum which is available at: (http://social.msdn.microsoft.com/Forums/en-US/dataservices/threads).
We look forward to hearing your feedback.
ADO.NET Data Services, Program Manager