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