You can save PS Touch projects in PSD format (which is Photoshop's file format). You then have to send your project from within PS Touch via email OR via Creative Cloud. You would then retrieve the PSD file and (of course) open it in Photoshop on the desktop.
Thanks. I see I can download either a jpg or png version.
A couple of clarifications,Warunicorn is mostly correct, PS Touch has its own native file format (or project format) PSDX. Its not possible to open Photoshop Touch projects natively in Photoshop CS4, it would require Photoshop CS5 or later. You could transfer a project saved as either jpg or png but this is a flattened version without layers.
This document describes the workflow for Photoshop CS5