TO ADD: @Christopher's answer does not work on API 7/8 (as per @Jonny's correct comment) IF you are using colours, instead of drawables. (In my testing, using drawables as per Christopher works fine)
Here is the FIX for 2.3 and below when using colours:
As per @Charles Harley, there is a bug in 2.3 and below where filling the list item with a colour causes the colour to flow out over the whole list. His fix is to define a shape
drawable containing the colour you want, and to use that instead of the colour.
I suggest looking at this link if you want to just use a colour as selector, and are targeting Android 2 (or at least allow for Android 2).