This seems to be a new problem introduced in the past week or two, and is not isolated to StackOverflow but I also experience it on dba.stackexchange.
In the mobile version of the site, on an iPhone (iOS 5.0.1), the top-right down-arrow is a no-op, but only when I've navigated to an individual question. The drop-down opens up the nav just fine if I'm on the home page or the Questions tab, just not on a question. Here is a screen shot to show what I'm talking about. If I load stackoverflow.com, I can use the down-arrow... it pops open the nav with links to my profile and recent comments etc. If I click on the first question, though, the down-arrow does not function on that page.
This is not a major show-stopper by any means, and there are several workarounds. So I'm not posting here to get answers for how to make my experience better while accepting this functionality gap, I'm more hoping it's a simple fix that the devs can implement (or at least explain).