Plumbing oversight

What is the name of your state? OHIO
Our house was renovated in 1986 and we just today (2006) figured out the source of a bad odor - an upstairs bathroom vent was not taken to the roof. The vent went up a wall but stopped an inch short of the ceiling. The pipe was not capped. At times it stunk badly but at other times you wouldn't notice it.
Do I have any recourse?

20 years is far to long for any reasonable expectation of a warranty. Since this has only caused a smell and not physical damage to the structure, I would suggest you just pay to have someone finish the job and put a roof vent in.