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The Maemo 5 documentation now includes also the GUI Design Template, which allows one to create high-fidelity image mockups.
Currently the GUI Design template is available in Photoshop format that has a fairly comprehensive library of assets – all fully editable.
As is usual with these things, this is not something you'd use e.g. for wireframes or as a replacement for pen an paper. This is more useful for situations where pixel perfectness is needed.