When trying to delete record types one has to log a call with Salesforce to delete a record type that was previously used as a default. This is because one needs to go in and manually update all the Profiles and deactive the record type from being active, but one cannot update the default record type of Chatter External User, Chatter Free User or chatter Moderator user.
It would be helpful if Administrators could change the default record types for those profiles themselves.
The error message I got when trying to delete a Contact Record Type (within Set | App Setup | Contacts | Record Type) is:
Only inactive record types can be deleted. Deactivate the record type to allow deletion.
This record type XXX cannot be deleted because the following profiles use this record type as default.
Chatter External User
Chatter Free User
Chatter Moderator User
Screen shot below: