mitchellh/vagrant — pull request #877 by bitprophet (2012-04-10 00:59:48+00:00)
Title: Don't use IdentitiesOnly under 'ssh -p'

Plain mode SSH shouldn't specify IdentitiesOnly, as that forgoes use of the local user's own SSH agent. Plain mode implies "use my local SSH setup, I just want Vagrant filling in the blanks re: VM port" so having one's agent work is pretty important :)