Here's the formula used by Android:
px = dp * (dpi / 160)
Where dpi is one of the following screen densities. For a list of all possible densities go here
It defines the "DENSITY_*" constants.
Taken from here.
This will sort out a lot of the confusion when translating between px and dp, if you know your screen dpi.
So, let's say you want an image of 60 dp for an hdpi screen then the physical pixel size of 60 dp is:
px = 60 * (240 / 160)