I haven't tried anything fancy with the onboard serial stuff besides using it what it's meant for (received my sheevaplug just yesterday).
I have a http://www.dealextreme.com/details.dx/sku.5859
(4$ usb-rs232 convertor, although mine is green
I plugged it into the not-mini-usb port after I read this, and it worked like a charm.
[ 3652.180000] usb 1-1: new full speed USB device using orion-ehci and address 2
[ 3652.330000] usb 1-1: New USB device found, idVendor=067b, idProduct=2303
[ 3652.340000] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 3652.340000] usb 1-1: Product: USB-Serial Controller
[ 3652.350000] usb 1-1: Manufacturer: Prolific Technology Inc.
[ 3652.360000] usb 1-1: configuration #1 chosen from 1 choice
[ 3652.410000] usbcore: registered new interface driver usbserial
[ 3652.420000] USB Serial support registered for generic
[ 3652.420000] usbcore: registered new interface driver usbserial_generic
[ 3652.430000] usbserial: USB Serial Driver core
[ 3652.450000] USB Serial support registered for pl2303
[ 3652.450000] pl2303 1-1:1.0: pl2303 converter detected
[ 3652.470000] usb 1-1: pl2303 converter now attached to ttyUSB0
[ 3652.470000] usbcore: registered new interface driver pl2303
[ 3652.480000] pl2303: Prolific PL2303 USB to serial adaptor driver
I connected the serial end of it to my garmin GPS unit, and gpsbabel was able to make sense of it, so it doesn't just show up in dmesg, it also seems to be functional.
I run a freshly installed debian squeeze from http://www.cyrius.com/debian/kirkwood/sheevaplug/index.html