I have heard this too, and I am really bored and can't sleep sooooooo...
from
http://mdc.mo.gov/conmag/1999/05/40.htm
Copperhead Myths
Copperheads smell like cucumbers. You may have heard someone say you always know when a copperhead is around because it smells like cucumbers. This is both true and misleading.
Yes, copperheads and most other kinds of snakes give off an offensive odor when molested, cornered or captured. This defensive odor, produced by glands at the base of the tail, is given off at will and may also be mixed with feces. To some individuals this musk may smell somewhat like cucumbers.
However, a snake has to have a reason to expel its musk. Thus, a copperhead at rest under a rock or alongside a log will have no reason to give off its musky defense. You could walk within a few inches of the snake and never know it's there.