I've noticed a lot more early resigning lately. And while I generally agree that people have the right to resign whenever they feel like it, I think the question of courtesy is not so easily answered--especially when it comes to Isotropic. Here are some of the principles that I subscribe to:
1. It is not rude to want to keep playing even after your victory is fairly well-assured.
There's more to playing the game of dominion than reaching the earliest point at which one player has a decisive advantage. The game is not over just because I got more Grand Markets than you. I play not just to win or lose, but to play. That means that if I happen to win the Grand Market race (whether out of luck or skill or the combination), I actually enjoy getting to use the strong deck I've built. That's part of the game! Just because turns 1-10 have left me in a much better position to win doesn't make it rude if I want to play turns 10-16 anyway. This is especially true if the turns are relatively short, so that playing to the end doesn't require a huge sacrifice of anyone's time.
A corollary principle is that it is not necessarily rude to continue racing for big VP cards when you could win by emptying piles. In the group I play with IRL, three-piling is seen as spoiling the game just so you can win. Obviously that's not true generally, and I don't think ending on piles is rude, but I also have no problem with a person who enjoys playing the "let's see who can get the most Provinces" game instead of the "let's see who can force the game to end while they have a narrow lead" game.
A caveat: This principle is only valid if the game is moving to a conclusion at some appreciable pace. If my stronger deck is some nightmare Goons + Watchtower engine where I can continue to rack up points for a long time without actually moving the game towards a resolution (not buying Provinces, let alone three-piling), then dragging the game out starts to feel like rubbing the other guy's nose in your victory--which is obviously bad form. Of course you might feel like my distinction is arbitrary (why can't we play the "let's see how many points I can get" game?), but that's the way I feel.
2. Resigning because you are at a disadvantage can be rude.
When my opponent resigns in this circumstance, it really comes across as their throwing a tantrum because they didn't get to win. Like they're trying to deny me the satisfaction of playing the stronger deck that I was fortunate enough to build. The other day I found myself in a position of having won the race to GMs and the race for Highways. I was set up for a couple massive turns, but the game itself would have taken only a minute or so longer to finish. My opponent's resigning had less to do with how important that minute was to him, and more to do with not wanting me to have the satisfaction of what could have been a very fun couple of turns to play. That's just rude.
Caveat: If you are in the likely-to-lose position, the polite thing to do would be to congratulate your opponent on his play and ask for permission to resign. If it's a long painful curse-filled game, the likely winner is likely to be all-too-happy to accept. If on the other hand the game is nearly over, or if your opponent is on the verge of a mega-turn, he may politely ask that the game continue for another turn or two. It would not be polite, of course, to insist that the opponent continue if the game was either long and painful or (returning to the Goons example) not remotely approaching an end-point.
Corollary to the Caveat: While asking to resign is always preferable to simply resigning, sometimes asking to resign is also preferable to playing on. I have played games to their conclusions that began with my missing out on Familiar the first three times that the potion came through my deck. Those games were painful, and I lost badly. But for as bad a time as I was having, my opponent was having fun. It sucks to get ten curses handed to you, but it sure can be fun to dish out that many. But once I knew I couldn't win, I asked if he wanted to play on. He did, so we did. That's the deal you make when you offer/accept a game of dominion. But if he had no interest in playing out a very slow game with a certain outcome, it was the least I could do to offer my resignation. Was his insistence rude? Maybe. But sometimes courtesy requires disregarding others' discourtesies.
Anyway, that's my take on the resigning issue. It boils down to two points: Dominion is about playing your deck, not just building one. And ask for permission before you resign, because your own immediate fun or frustration is not all that matters.