I've told a select few people. We have some friends a few states away (from before I moved for a job) who were completely on board. They were about my age, and they saved up throughout their 20s to own their house outright. I suspect they'll be FI by 40 with rental properties.
I've told my older brother, and he seemed interested, but didn't ask too many questions. He recently told me his plan is to have $1 MM saved up by the time he's 50-55, and I told him it's completely doable. We don't talk too much about money, so that's more than he's usually willing to offer. I have not told my younger brother or older sister. I just can't imagine either of the giving a flip about it. They are both "decent" savers. I'm sure they save about 20%, maybe more, but they're too into new cars, boats, and big houses to get interested in early retirement.
I've also told my Dad. He retired in his early 50s. He was super-frugal when we were young, mostly because he had to be (four kids, after all). But he rose up the ranks and has a hefty pension with spending to match (To be fair to him, his largest expense is caring for my mother, who has Alzheimer's, to the tune of $7,000/month. But he can still afford all the toys, the big house, and plenty of eating out). He was taken aback when I told him I would be retired by 40, and asked me how. I explained my plan, and he's never had a negative thing to say about it. He's knows what I earn, and I suspect he's relieved to know that I don't live the way I do because of debt problems.
Other than that, I've told a couple of trusted co-workers at my previous employer. Both were naturally frugal, and very interested. I passed on MMM's link and figured I'd let nature take it's course. Haven't heard from them in about a year. I suspect both of them will end up at odds with their spendy spouses, in all honesty, but maybe they'll at least max out their 401ks as a result.
Finally, I was hanging out with some friends from college earlier this year, and I amazingly discovered that one of the couples were MMM disciples and all aboard the early retirement train. It was pretty exciting, and we discuss FIRE plans pretty much any time we get together, which isn't nearly frequently enough.
Other than that, I don't think a soul on the planet knows about my FIRE plans. I stay pretty tight-lipped about it, mostly because no one cares.