Failed to create field: Field type myfield is not installed properly

You have seen this error in MOSS 2007 and it's the same in SP 2010:

"Failed to create field: Field type is not installed properly. Go to the list settings page to delete this field."

This can happen when you activate or deactivate a feature that includes this field. The message is not helpful. There are plenty of discussions about this and apparently a gazillion scenarios that this could occur. In my case, it's because myfield is a custom field inherited from SPFieldMultiColumn. And I didn't include the class (and the assembly) in the deployment. Hence when it goes to create or delete the field, there's nothing other than the metadata in the XML field definition.

So at least one scenario cleared up.