That is an awesome example. I'm absolutely serious about stealing that from you (with your permission).

Do you think this presents a serious problem for utilitarian ethics? It seems like it should, though I guess this situation doesn't come up all that often.

ETA: Here's a thought on a reply. Given restrictions like time and knowledge of the names of large numbers, isn't there in fact a largest number you can name? Something like Graham's number won't work (way too small) because you can always add one to it. But trans-finite numbers aren't made larger by adding one. And likewise with the largest real number under thirty, maybe you can use a function to specify the number? Or if not, just say '29.999....' and just say nine as many times as you can before the time runs out (or until you calculate that the utility benefit reaches equilibrium with the costs of saying 'nine' over and over for a long time).