I don't know exactly how I'd quantify it, but seeing evidence that a broad range of substantial projects are using it in production and/or a substantial proportion of practising software developers were at least aware of its existence would be a good start.
What language would you compare it to, and where was that language at 2 years old? What was Java adoption in 1998? What was Scala adoption in 2006? What was Python adoption in 1996.
Hardly anyone uses languages that immature for real work. None of the languages you mentioned could reasonably have been called mainstream at the times you mentioned. Scala still can't today.
it seems to have decent penetration on both ends of the spectrum, large enterprises and tiny start-ups are choosing it (http://code.google.com/p/go-wiki/wiki/GoUsers)
That's an impressive list for a language so young, but when you start reading the supporting links, it turns out that quite a few of the "Go users" are minor supporting projects. For example, the gov.uk project is vast, but it appears Go was only used to write a routing tool; the main sub-sites are written using languages like Ruby and Python. Another popular project I noticed is GitHub, but it turns out that the cited blog post is about "a really simple, low traffic service to dip our toes in the Go waters."
There's nothing wrong with these early experiments, of course, but if you want to claim mainstream status, get back to me when all of a site like GitHub or gov.uk is written in Go, or when the big G themselves are using it for something like the back-end of Google Mail or Google Search rather than the Santa Tracker.
If you want to use GitHub as a barometer of language popularity
Well, I don't think GitHub is a particularly useful barometer, but OK...
Go is tracking darn close to Scala, despite being 8 years younger.
Enough said.
Sorry if this post comes across as a bit snooty, but it's hard to reply entirely seriously when almost everything I'm replying to is making my point for me. I have absolutely nothing against putting new languages out there or even just new language features that might be interesting to explore. But the idea that a language that is mostly an incremental development of what we already have in abundance and that is barely out of prototype stage will somehow take over the world for the next decade is just silly, and it's still silly even if the language happens to be promoted by Google.
