@warren - Thanks for your contribution. The bit about being thrown out of the house was an exaggeration but they are both quite firmly anti-smoking. Although I don't live it at home for most of the year - I'm in college elsewhere - I do rely on them for financial support. I used to smoke cigarettes (the pipe is really helping me come off them, thank God) and I think they knew that in a wink-wink sort of way without that knowledge ever being explicitly confirmed. I know for a fact they wouldn't be a bit impressed if I admitted openly to the cigarettes, but I think a pipe can, and should, be thought of differently - but convincing them of this might be the hard part.
I don't have any firm reason for wanting to tell them beyond simply not having to feel furtive and conspiratorial about a hobby that is supposed to be about pleasure and relaxation. (As I mentioned, even if I did tell them, I wouldn't anticipate getting any TAD/PAD presents any time soon, but it would be a bonus :P) When I'm home, for example, over the holidays, I don't fancy the idea of nipping out the back with my pipe in the half-hour that my mother is at the shops. If you have to stress out about it, what's the point?