The one serial connection on the Arduino board will not allow a configuration like the one you are describing.
The standard rs232 port is really not meant to be used for anything but two devices communicating with each other, there is no way to address multiple devices built in.
One workaround could be to use the softe serial (see the playground). Then use the standard serial connection on the first Arduino board to communicate with the PC, the softserial to communicate with the standard serial on the next Arduino board and so on.
But this setup will not allow you to adress each board directly. Each board would have to read the serial data, check if he was the intended reciever, or otherwise send the data to the next board, and so on. This might add som undesired overhead / latency, depending on how "real time" your devices need to be controlled.
If the devices you are going to control are located not to far apart, a better solutuion might be to find a way to multiplex the PWM outputs, so you would only need one Arduino board to control your devices. There's an IC that will allow you to do that.
Or you could try to look into RS442 this forum topic http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1195139560 might be of interest.
MikMo