The 2.2 upgrade notes state:

mongodump Output can only Restore to 2.2 MongoDB Instances

If you use the mongodump tool from the 2.2 distribution to create a dump of a database, you may only restore that dump to a 2.2 database.

See: SERVER-6961 for more information.

The summary of the issue as described in the upgrade notes is incorrect. SERVER-6961 has the correct summary. Specifically, this issue is specific to interoperability of different versions of the mongodump/mongorestore tools; it is independent of the server version.