It's too easy to mistakenly tap upvote and downvote buttons on the iPhone. I suggest asking for confirmation for any kind of voting action performed on the iPhone's Mobile Safari.
I think the answer here is figuring out what the use case is for mobile browsing of this site. If it's the full experience including answering questions, then I would agree with the arguments that people tend to write long posts, and thus the effectiveness of mobility is less.
My thought is that a mobile version of the site should be created to address this issue. It is mostly a presentation issue, and could easily be developed by the SO team in a reasonably short time period.
However, I don't think the amount of use this would get justifies creating this specialized version.
In a lot of ways I want to agree with this...it can be frustrating...
But on the other hand I have to wonder if this is a case of the solution being too much bother to develop when you could simply say to the iPhone user "Hey...maybe you should zoom in on your screen a bit more to separate the up and down vote arrows."