This may not a “stat”, but take a look at Twitter Bootstrap that is starting to be used a lot.
I guess this depends on the design, which might differ each build. It also depends if your using 100% fluid or changing on breakpoints.
I tend to use some of the following, generally the large sizes don't change much at all between breakpoints:
No point trying to cover all the different android etc resolutions...there are so many.
Also sometimes the breakpoints don't trigger on the actual size due to the scrollbar, which each browser has different rules for. Might be wise to cut off 20px per @media to make sure they trigger. I sometimes swap the min-width:320px for a max-width: 480px, as under 320 nothing might be displayed.