Comment 1:

I'm worried that will lead to people write racy code such as the following:
var wg sync.WaitGroup
for i := 0; i < N; i {
    wg.Add(1)
    go func() {
        ...
        if wg.Done() { 
            close(out) 
        }
    }
}

Comment 2:

You are right!

Status changed to WorkingAsIntended.

Comment 3:

This frequent can be remedied with a function:
func Spawn(N int32, fn func(id int), autoclose ...interface{}) {
    var counter int32 = N
    for k := 0; k < int(N); k += 1 {
        go func(k int) {
            fn(k)
            if atomic.AddInt32(&counter, -1) == 0 {
                for _, ch := range autoclose {
                    reflect.ValueOf(ch).Close()
                }
            }
        }(k)
    }
}
And the usage example: http://play.golang.org/p/8M156uFYPW.
Alternatively, the last parameter can be a callback to cleanup:
func Spawn(N int32, fn func(id int), whendone ...func()) {
    var counter int32 = N
    for k := 0; k < int(N); k += 1 {
        go func(k int) {
            fn(k)
            if atomic.AddInt32(&counter, -1) == 0 {
                for _, fn := range whendone {
                    fn()
                }
            }
        }(k)
    }
}