There are a few functions to help you: [wiki]love.window.fromPixels[/wiki], [wiki]love.window.toPixels[/wiki], and [wiki]love.window.getPixelScale[/wiki].
Currently all graphics, mouse, and touch input units are in pixels rather than pixel density-scaled points, which makes dealing with it a bit of a pain. I plan to make it all a bit easier to handle for LOVE 0.11 (which isn't out yet).