Hey, yes, in Fastmail it is properly implemented - it behaves just like the iOS email client, or others: you have to swipe all the way (either to the left for archive or to the right for delete), but if you swipe back, or if you simply release without going the whole way, it will just cancel the action. And you get the swipe animation, so it's intuitive. Which is not something you see often in mobile web interfaces :-)
