CRM 3.0
When you export records from CRM 3.0 to Microsoft Excel only the first 10,000 rows are exported. Unless you scroll to the end of the spreadsheet there isn’t any kind of indication that a limit has been reached. The following post from Microsoft walks you through the steps necessary to increase the limit.
http://support.microsoft.com/kb/911395
CRM 4.0
To change this in CRM 4.0:
1. Log-in to the SQL Server where the _MSCRM database is stored
2. Open the OrganizationBase table
3. FInd the Column: MaxRecordsForExportToExcel
4. Change the value from 10,000 to the desired value
5. Committ the change to the database
MayankP's Blog 