I only noticed it once when Snowwolf mentioned it, but I'd like to restate it: ALWAYS stop and ask someone if they need help if they wipe out. It may interfere with your run, but they could have seriously hurt themselves and don't know it yet. I cracked my head once in a bad fall and didn't realize I was hurt bad until I tried to keep going. I had blurred vision and I lost my balance completely. I had to crawl off the slope and behind some snow fencing and no one stopped and even ASKED if I was ok. I laid there on my back for 20 minutes and still no one stopped.
Even if it means stopping your awesome run, please, if you see someone take a nasty spill, just ride by and ask, "You ok?" Even if they say they are ok, make sure they can stand up and help them get their gear if they pitched a yard sale.
It may not be a rule, but it is common decency and if you can't show another person some common decency don't expect any in return. I'm sure most, if not all, of the people on this board would help because I haven't met anyone on here that seems like a total wanker, so I'm probably preaching to the choir. Still, if you're just browsing the board, this is one of the most important threads to read in my opinion. Snowboarding without knowing etiquette is like driving without knowing what all the signs mean.