every time you perform i/o, the current goroutine yields to the scheduler.
If you want to manually yield to the scheduler, you call runtime.Gosched() and the calling goroutine yields. If you want to lock a goroutine into a specific thread, you call runtime.LockOSThread(). Everything is single-threaded to start until you call runtime.GOMAXPROCS(), but that's intended to go away in the very near future. So... I'm not sure what you mean by sealed away; the Go runtime does the sensible things that it can do automatically, but you still have the ability to change the scheduling behavior if you really want to.
what do you mean by "doesn't offer a select primitive"? I'm not familiar with your definition of select, because Go has a select keyword for concurrency control, and I'm under the impression you're talking about select as its defined in EventMachine, could you elaborate?