I wrote this as a reply to someone else above:
I think the dust speck argument would be better if first framed as a preference for yourself:
If you were going to live for 3^^^3 lifetimes, would you like to be tortured for 50 years in exchange for one fewer dust specks in your eye during each of those lifetimes.
I think most people would answer no. So the debate has nothing to do with morality in general. Instead the debate is about Scope Insensitivity (http://lesswrong.com/lw/hw/scope_insensitivity/), that our emotional reaction to things isn't diminished by even infinitesimally small quantities (see also Useless Medical Disclaimers http://lesswrong.com/lw/h4/useless_medical_disclaimers/).
I think I can make that position seem more reasonable if framed another way: Would you risk a 1/(3^^^3) chance at 50 years of torture, in exchange for getting rid of one dust speck out of your eye? I think most people would say yes. You couldn't even get out of bed in the morning if you weren't willing to take even incredibly small risks.
There is a much higher probability than 1/(3^^^3) that you could get in a car accident with injuries that cause 50 years of incredible pain, yet you will still probably risk driving for even trivial things.
And if you lived for 3^^^3 lifetimes and took this risk each time, you would likely suffer 50 years of torture during at least one lifetime.