oh jamie, i am laughing only b/c my daughter is going through a "fart" stage. we were at a 50th birthday party recently with lots of 'old fart' decor and talk... and it went from there.
to give you some background on my parenting, i am pretty on top of things for the most part. when something happens, we address it, don't make empty threats 20+ times in public or at home for that matter, ect.... we are not religious, but my family is... so i don't allow her to say "oh my god" instead she can say "oh my goodness," or "good for goodness sake" just like santa :o)
hearing her come up with all by herself... "old mcdonald had a fart, e i e i o. and pfft pfft here and a pfft pfft there.... oh my. i had to burry my face behind a pillow to hide my laughter so i was not encouraging her until i could regain my composure and address the situation ....
but sometimes i think we also have to step back as parents and see things through our childrens eyes. it is our job to teach them and guide them, but having a little fun along the way is ok too. so i took the pillow away from my face and laughed with my daughter. when things calmed down, i explained that it is not polite to say fart, or sing fart songs in public... but that once in a while we can have silly time, or silly song time at home. and we do. and it's fun! we change the words to lots of songs (not just fart related, lol)
i dont see darn it as a huge deal, but if you introduced a new phrase like fiddle sticks or some that others have suggested i am sure he would pick up on it quickly and use that instead. it didnt take my daughter long to not say oh my god anymore.
my nephew walks around saying "what the?" he doesnt say hell, but to me it just sounds icky from a little boy. as long as you don't let it get to that extreme, i really wouldnt worry about it or beat yourself up over it. it sounds like you already have the right idea of what to do. good luck! darn it! lol.
oh and ps.... i was reading through some other posts... and i respectfully disagree with the idea of ignoring unwanted behavior and it will go away, no matter what the situation is. in my experience, with my daughter, and other children close to me in my life... ignoring doesn't work. if you teach children and guide them... they will listen as long as you are consistent with it. ignoring early on only made my daughter try harder to gain my attention with whatever poor behavior she was displaying at the time. i learned very quickly that teaching her, re-directing her, guiding her... worked much better. after all, that is our responsibility to our children as their parents. that is just my own personal opinion.