I recently wrote a blog on using the import tool from CodePlex to import records http://blogs.msdn.com/dynamicscrmonline/archive/2009/05/15/microsoft-dynamics-crm-import-tool-on-codeplex.aspx. Although its called the import tool it's based on the export tool and does allow you to export data out.
I wanted to point out a few tips. You an use an csv file to upload the data natively in CRM using the Import Wizard. You an also export data using the advanced find tool. The caveat is that you need to include the object id of the record in order to update the record during import. Normally, the import creates a new record rather than update an existing. In order to have a the import wizard do an update instead of a new create on, the GUID of the record needs to be in the first column.
The trick is getting the object id out. The tool mentioned above does so and so does the Bulk Export tool.