While this tweet posted above from NDP HQ distorts the historical record, as the Greens actually opposed it before the NDP and have in far stronger terms, it is still very good and very important news, given that the NDP is the Official Opposition.
There were rumors this announcement was coming all day. While Tom Mulcair himself still has yet to say it, it would appear to be the newly stronger NDP position.
This shows the importance of keeping up pressure against opportunist and facile attempts to appear to play both sides and of standing up against the rhetoric and excuses of the partisan hacks.
No longer is solely a highly defensive (at the time) Randall Garrison saying it at a town hall meeting.
It would appear highly likely that The Greens played a very big roll in pushing the NDP's hand with petitions and memes like this:
At any rate, better late than never!