i'm having the
SAME problem now with my 7yr old cat, last
friday he "marked" his territory on my living room curtains, its the most disgusting smell in the world. i had a carpet cleaning company come in on
saturday morning and clean the area, i also cleaned my curtains and sprayed some product that claims to "prevent pets from marking the area" and
SUNDAY MORNING my son and i were sitting on the couch watching The Incredibles and the cat went right back that same spot and "marked" it again RIGHT IN FRONT OF US!!!
i can't keep paying to get the carpet cleaned so i bought a carpet cleaing wet/dry vacuum thingy this morning but i can't keep cleaning this EVERYDAY. i don't know what to do, it's horrible. i'm worried about my yorkie Lefty picking up this bad habit from my cat.
my cat is 7yrs old, neutured and only started doin this friday, out of the BLUE. what causes this? and how do you stop it? 
any advice is MUCH MUCH MUCH appreciated . . .