I am actually doing exactly the same thing as you, I am using a sheeva with a USB to 485 FTDI232BL
Bus 001 Device 005: ID 0403:6001 Future Technology Devices International, Ltd FT232 USB-Serial (UART) IC
I used the installer to set up ubuntu on an SD card, did an apt-get update/upgrade and then installed an up to date kernel ( Currently using Linux ubuntu 2.6.33)
When you attach the USB 485 it will appear in /dev as ttyUSBx (Where x is the next availible number).
I use a udev rule filter to symlink the /dev/ttyUSBx to /dev/usbserial so my C application can check and use /dev/usbserial instead of trying to find out which ttyUSBx is the 485 device
You can then treat it like a normal serial port, my USB to 485 automatically does the direction line switching not sure that's a function of the chip or my board so be aware of that.
Installing a new kernal was easy, I just did :
chmod 777 README-2.6.33
. README-2.6.33 --nandkernel
That downloads a script, makes it executable then runs it tell the script we want to flash the kernal in Nand.