Verite: since you asked...
Here's my experience: like some other posts, I made the mistake of telling a doctor I smoke. This resulted in getting asked "how the pot smoking is going" every time I went in. As Soilman said, a good doctor should ask this question, and in general he should ask ALL of his patients (including the elderly) if they smoke, drink, or take recreational drugs. But that doesn't mean he won't be judgmental, even if not outwardly. (As a side note: it is a common assumption that alcoholics and "drug" users don't give a shit about their health, so doctors don't fight AS hard for your health when it comes to fighting insurance companies, sending you to expensive specialists, etc.)
After I spilled my guts and regretted it for the next 2 years, I moved (from a big city) to a small town where I basically worked with town's few doctors. My new doctor wanted my previous records and asked me to sign a privacy waiver to get them from the old doc. Here's what I did: I told the new doc I got a copy of my chart before I moved. Then I ran home like a scared little girl and called the old doctor and requested a copy of my chart. Under the HIPAA law (Health Insurance Portability and Accountability Act) every patient has a right to a copy of their own records, sometimes with an added copying fee (BTW: this same law is what protects your privacy as a patient, its worth a wikipedia read). I got the chart and basically looked for the part where my pot use started getting documented. I took out the pages that made reference and basically cut out the last couple of years. I took the chart to the new doctor and (out of paranoia mostly; should have seen if he cared first) filled in with some BS about how I didn't have insurance for the last couple of years, so I don't have recent records, could he please do a complete blood workup to ease my mind.
This was generally a shitty situation, because any change in my health or medications that took place after this I had to put in the context of "I had to see random doctors who didn't charge much, so I don't have the records, please take my word for it". If you take pain or anxiety meds, or if you have a serious medical issue, this is really bad for your health because a new doc needs to know everything.
If you recently told a doctor about your smoking, I would tell him you quit for any number of reasons so that when you do get your records, you have to take out less pages.
This is why its bad to say anything in the first place: most of us don't see the same doctor for our whole lives and every time you change doctors, you never know who the new one is going to be. Same goes for insurance companies: For example, blue cross wants ALL of your records to sell you insurance and "drug" use is an easy way to jack up your rate; could even be seen as a "pre-existing condition" since you might want to go to rehab or something
But as some people here have experienced, there are those good doctors who actually know the science or are willing to put the patient's needs first by not writing the stuff down in a legal document (your patient record is considered a legal document).
Lastly, if I ever got a life-threatening illness, I would be up front.