I bought a "Digitus Accessories USB 2.0 - RS232 Cable" and wondered if it would run under linux. Reading around on such sites as linux-usb.org I found that yes, it is supported. Quite well as it seems. This is the information I gathered:
What you need for 2.6.x is usb serial support (CONFIG_USB_SERIAL=m in .config) and ftdi support (CONFIG_USB_SERIAL_FTDI_SIO=m). I think for 2.4.x (> 2.4.19) you need serial as a module. Then, when you plug it in, these modules should be loaded for 2.6.x:
8250 27968 0
serial_core 19808 1 8250
ftdi_sio 26596 0
usbserial 25740 1 ftdi_sio
Then, when you plug it in, this is what dmesg tells you:
hub 2-0:1.0: new USB device on port 1, assigned address 3
usbserial 2-1:1.0: FTDI 8U232AM Compatible converter detected
usb 2-1: FTDI 8U232AM Compatible converter now attached to ttyUSB0 (or usb/tts/0 for devfs)

This is for kernel 2.6.2(rc3 to be precise). So, your new serial port is at /dev/usb/ttyUSB0.

Further information, for searches and such:
lsusb:
Bus 002 Device 003: ID 0403:6001 Future Technology Devices International, Ltd 8-bit FIFO
Here you see what FTDI stands for ...
This is the linux-usb.org page regarding the chip inside of the adapter.
This thing was rather cheap where I got it: 19 Euro which equals to roughly 24$. The pages of the "producer" are www.digitus.de and www.digitus.info. This thing also comes with a DSUB 9 to DSUB 25 connector.
The model I have is: DA-70128 - Rev. 1.0
My image of the thing

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