how to listen to N channels? (dynamic select statement)

You can do this using the Select function from the reflect package:

func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool)

Select executes a select operation described by the list of cases. Like the Go select statement, it blocks until at least one of the cases can proceed, makes a uniform pseudo-random choice, and then executes that case. It returns the index of the chosen case and, if that case was a receive operation, the value received and a boolean indicating whether the value corresponds to a send on the channel (as opposed to a zero value received because the channel is closed).

You pass in an array of SelectCase structs that identify the channel to select on, the direction of the operation, and a value to send in the case of a send operation.

So you could do something like this:

cases := make([]reflect.SelectCase, len(chans))
for i, ch := range chans {
    cases[i] = reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(ch)}
}
chosen, value, ok := reflect.Select(cases)
// ok will be true if the channel has not been closed.
ch := chans[chosen]
msg := value.String()

You can experiment with a more fleshed out example here: http://play.golang.org/p/8zwvSk4kjx

You can accomplish this by wrapping each channel in a goroutine which "forwards" messages to a shared "aggregate" channel. For example:

agg := make(chan string)
for _, ch := range chans {
  go func(c chan string) {
    for msg := range c {
      agg <- msg
    }
  }(ch)
}

select {
case msg <- agg:
    fmt.Println("received ", msg)
}

If you need to know which channel the message originated from, you could wrap it in a struct with any extra information before forwarding it to the aggregate channel.

In my (limited) testing, this method greatly out performs using the reflect package:

$ go test dynamic_select_test.go -test.bench=.
...
BenchmarkReflectSelect         1    5265109013 ns/op
BenchmarkGoSelect             20      81911344 ns/op
ok      command-line-arguments  9.463s

Benchmark code here