It was discussed back in 2011
 https://bitcointalk.org/index.php?topic=8392.0

The patches were submitted in late 2012: https://github.com/bitcoin/bitcoin/blame/master/src/script.cpp

The protocol specification was updated in April 2011:
https://en.bitcoin.it/w/index.php?title=Protocol_specification&oldid=7624
Edited on 24 April 2011:
"Signatures use DER encoding to pack the r and s components into a single byte stream (because this is what OpenSSL produces by default). "

MtGox is a bunch of liars (should make this a signature, probably).