I heard someone claiming this, but as I understand the blood takes longer than that to go from your lungs to your brain. So, is it true?
Under 10 seconds is about right. Lungs to heart to brain, all in a few heartbeats.
It takes longer for IV drugs to take effect, as they have farther to go. And they act in 10 seconds or so, as any IV heroin user will tell you.
As a very occasional smoker, I can assert that it takes about half that amount of time.
Anecdotal evidence: I have never been a regular tobacco user. In college, I tried dipping some Copenhagen once. I put a pinch between my lip and gums and within just a few seconds (no more than ten) I felt an intense–albeit brief–high.
More anecdotal evidence : I still remember my first smoke. Made me dizzy, more than a little bit nauseous and gave me palpitations by the second puff. 10 seconds sound like more than enough.
It should be about the same for smoked marijuana right? In my experimental days I would feel pretty much instantly blazed after taking a big bong rip. (although I always practiced a very slow inhalation/exhalation as I was told is the optimal method so I spent a good 20-30 seconds just taking and exhaling the hit)
The only faster delivery methods for a drug than inhalation would be either snorting it (if it can be properly absorbed thru the olfactory nerve, and not just absorbed by nasal mucosa) ,injecting it into the carotid artery or pumping it directly into the cerebrospinal fluid surrounding the brain.
The carotid method is frowned upon as being rather deadly, the CSF method is used in some clinical circumstances. And the snorting of some drugs does lead to direct absorption into the brain via the olfactory nerve, but is not usually used in a medical setting.
What drugs get absorbed by the olfactory nerve? I vaguely recall reading something about that a while back but I never thought about it being a faster drug delivery method than inhalation.
How fast do swallowed powders effect you?
One of my coworkers used to have goody’s Aspirin powder in here desk, and It really seemed like that stuff made you a little dizzy within seconds. I always wondered if it was pschosomatic, or if it was just slipping right into my blood like it seemed.
I’ve heard the claim that loose liquids or powders (as opposed to the “gel caps” that encapsulate liquids or powders for many pills, these days) can be partly absorbed into the bloodstream through the mucous membranes under the tongue and around the mouth and throat. I’m not sure about aspirin powder, but I believe it’s true for at least some drugs.
I believe the usual time of onset for a swallowed medication is from 15-45 minutes.
If any of my information above is incorrect, I hope the usual suspects will correct me speedily and thoroughly. 
Morphine is one drug that can enter the CNS via the nose. This paper lists a few others.
(beware, PDF)
My money’s on either psychosomatic, or vagal/gastrocolic reflex from stomach distension due to ingestion of fluids. Or both. Or other.