When you import organizations into an existing environment, there is the possibility that the import could fail. This post should help you to solve the most common issues.
With large organizations, the most likely problem is that the import process runs into a timeout when it updates the user mapping. Update Rollup 8 contains a fix for this problem which has to be enabled manually. See http://xrm.ascentium.com/blog/crm/Post768.aspx.
Another common issue is that the import fails with the following error
ExecuteNonQuery requires an open and available Connection. The connection’s current state is closed.
This error is likely caused by an issue in the .NET 2 Framework. See this article of the EMEA CRM Support team for a description
Another cause of this issue are reports which could not be published to the report server by any reason. For example, reports which are spiced up with custom assemblies cannot be published without the referenced assemblies. The import will fail when it tries to publish such a report. In this case enable the tracing, which generates a trace file for the mmc-console. With help of this file you can identify the report which is making problems. To solve the issue, you can either delete it directly from the database (unsupported) or you delete it prior to the backup of the originating system. After the successful import, you can import the problematic report again.
See this thread in the CRM Deployment forum for further description http://social.microsoft.com/Forums/en/crmdeployment/thread/1336bb5d-a119-44e3-83dd-1f557edccc31