I'm upgrading a publishing site from 2007 to 2010, the upgrade completes with errors in the upgrade log, all of which relate to a particular custom field type. An example of one the errors is here:
[powershell] [SPSiteWssSequence2] [ERROR] [07/11/2011 16:21:42]: Feature upgrade incomplete for Feature 'PublishingWeb' (Id: 'xxxxxxxxxxxxxxxxx') in Web 'http://xxxxtest01'. Exception: Field type YourCustomFieldType is not installed properly. Go to the list settings page to delete this field.
The field type is being used in the site content so we can't go ahead and delete it (although it is tempting) - are there any known issues we could look into which affect the upgrading of field types?