It was documented (Re: Warning: Cisco RW community backdoor. )

What is the difference between a bug and a feature, the feature is documented.

Protocol Limitations

The following are some known SNMP limitations.

ATM messages must be formatted according to SNMP version 1,
   not SNMP version 2.

ALL SNMP messages will use the community name ILMI.

In all SNMP Traps, the agent address field always has an IP
  Address value of

The supported traps are coldStart and enterpriseSpecific.

In all SNMP traps, the timestamp field contains the value of
  the agent�s sysUpTime MIB object at the time of trap generation.
  In all of the standard SNMP traps, the enterprise field in the
  Trap PDU contains the value of the agents sysObjectID MIB object.
The size of messages can be up to 484 octets.

Those darn ATM people again. Screwing up a perfectly good protocol
for their ATM UNI ILMI stuff.