I also have this same problem any help would be great.I had this working in Karmic, have installed Oneiric and am now in a mess.
I can do everything up to:
When I go into the CUPS interface (CUPS 1.5?) to add a printer I dont get the option to add a new custom printer, only to add printers from a defined list. One of them is a Lexmark 2500 series, however if I select that I can't select "Lexmark USB Backend #1 (Lexmark X2500 series)" anywhere.
Can anyone shed any light!?