I installed my Xubuntu 8.04 system on a Acer Extensa 5220 from a USB stick.
I too have a line with a usb device mounted as a cdrom.
Solution: I commented out the line with "/dev/sdb1" in it AND in the line of "/dev/scd0" changed "/media/cdrom1" into "/media/cdrom0".
I installed my Xubuntu 8.04 system on a Acer Extensa 5220 from a USB stick.
I too have a line with a usb device mounted as a cdrom.
Solution: I commented out the line with "/dev/sdb1" in it AND in the line of "/dev/scd0" changed "/media/cdrom1" into "/media/cdrom0".