In this article Jeff Handley gives you valued information that might help you avoid the DomainDataSource DataPager PageSize Pitfall.
During the PDC release milestone of RIA Services, we discovered an error from DomainDataSource when using AutoLoad="True" and when using a DataPager bound to the DomainDataSource’s Data property. We found that when the PageSize property is specified on the DataPager and not on the the DomainDataSource itself, an exception would often (but not always) be thrown from DomainDataSource, indicating that a load could not be invoked while another load was in progress. The reason for this will actually interest you most if you use AutoLoad set to False.