I want to send signals via serial port using the JavaComm API classes on an Android device, and here is how I imagine it:
1- the Android device would be: Archos 3.2
As noted above, you'll need to come up with device drivers for whatever USB<->Serial device you have. Many of these devices use the Prolific PL2303 chip:
http://www.qbik.ch/usb/devices/showdr.php?id=175
which the stock linux kernel has support for, and has had for a long time. I've used this and it works fine, so I would recommend you stick with USB<->Serial devices that use this chip.
Beyond that, with udev these devices normally show up as /dev/ttyUSB#. How that happens with Android, I don't know, so you'll need to figure that out.
Finally, you will need to build the RxTx native code. This code is pretty crufty - I had problems building it on a fairly modern Angstrom, and I was not trying to cross-compile at all.