I've recently started saving homescreen links to various Stack Exchange sites on my iPhone. All of them look great, but the Super User icon, having dark gray on dark gray, looks like it's only half of the logo when viewed from a regular distance on the small phone screen.
Here is a screenshot of various logos, blown up to show the SU logo with more contrast.
(Just like cereal boxes!)
It's certainly not an urgent issue and it isn't anything that impedes the site's use, but I believe that if the Super User iPhone Home Screen Icon had a white background it would be more iconic and certainly more pleasing to the eye.
Note that nearly a year later, after Arqade (Gaming) and Role-Playing got new icons, the Super User icon is still the same.