I feel[1] that a smart/talented C/C++/anything developer can go from someone who has never seen or heard of golang to a proficient and productive Go developer in a matter of a few weeks, maybe even _days_, if not less.
That's how long it takes to go through the following materials (and fav some for later reference) and play with the language a bit.
http://www.youtube.com/watch?v=ytEkHepK08c
http://commandcenter.blogspot.com.au/2012/06/less-is-exponen...
http://www.youtube.com/watch?v=XCsL89YtqCs
http://golang.org/doc/code.html
http://golang.org/doc/effective_go.html
http://golang.org/pkg/ - use as reference
http://www.youtube.com/watch?v=f6kdp27TYZs
http://talks.golang.org/2012/splash.article
http://golang-examples.tumblr.com/
And a some more similar things that you can mostly get to from golang.org site. The beauty of how concise the language and even its website are, is that you can literally just go through everything there one thing after another.
[1] This is my personal opinion based on playing with go the last few weeks/months. I'd love to verify this theory. It's not yet the primary language in which I do things in (I use C++11 atm), but for all my side tasks[2] it proved to be indispensable. And I found it very easy to pick up. I can't wait until I start doing all my work in Go, that will be a true test of its productivity efficiency.