When importing solutions from other systems (particularly advanced systems with tons of custom entities) you will receive an error message not allowing the import of the solution. If you download and open the error message, in the XML you will see that optionset values (picklists) fail. Even more specifically, left over optionsets from CRM 1.0/1.2 releases as well failing due to NULL labels.
The fix for this issue was a manually workaround deleting the values from the XML and reimporting the XML for the option set minus only. Thanks to Microsoft Support, we now have a hotfix available for those who cannot wait for the upcoming CRM 2011 Rollup 7. The KB Article #is 2651616 and has not yet been published at this time.
You will have to request this download from Microsoft support directly.