(See http://standards.ieee.org/develop/regauth/ethertype/.) Some IETF protocol specification make use of Ethertypes. All Ethertype requests are subject to review by a consultant to the IEEE RA followed by IEEE RAC confirmation.
Since Ethertypes are a fairly scarce resource, the IEEE RAC has let us know that they will not assign a new Ethertype to a new IETF protocol specification until the IESG has approved the protocol specification for publication as an RFC. In exceptional cases, the IEEE RA is willing to consider "early allocation" of an Ethertype for an IETF protocol that is still under development as long as the request comes from and has been vetted by the IESG.
To let the IEEE RAC know that the IESG has approved the request for an Ethernet assignment for an IETF protocol, all future requests for assignment of Ethertypes for IETF protocols will be made by the IESG.
Note that playpen Ethertypes have been assigned in IEEE 802  for use during protocol development and experimentation.
 IEEE Std 802a-2003 (Amendment to IEEE Std 802-2001).
IEEE standard for Local and Metropolitan Area Networks:
Overview and Architecture -- Amendment 1: Ethertypes for
Prototype and Vendor-Specific Protocol Development.