ADMINS: I couldn't decide if this message belonged in "interfacing" (seems to, to me) or user circuits (it's that, too). Relocate/crosspost as you see fit :-)
I had an idea that it might be possible to switch the TTL-level serial lines of an Arduino between two (or more) other devices. I got a 4066 (cmos) quad bilateral switch to try it out, and it seems to be working -- and rather well, at that.
The 4066 "switches" (input/output go either direction) actually have SOME resistance (about 270 ohms, at 5V), so there is a little bit of a voltage drop through the device. But the output was still sufficient to drive the serial-input chips that I tested with.
My simple demo circuit switches an Arduino TX signal between either of two outputs, but adding the other two gates from the 4066 would allow the RX line to be switched, too, of course.
I can't see any reason that something like a binary decoder (such as 74138 3to8) couldn't be used to control additional 4066 switches, and allow the use of fewer Arduino pins for "channel" control.
Anyway, I've put the circuit up on my website http://sites.google.com/site/adifferentlemming/Home/projects/arduino. You may or may not want to download the Eagle lib I created that includes a schematic symbol for an Arduino (with a PCB footprint that matches the Diecimila headers).