Click edit/preferences/unit and rulers. Make changes there.
I think it only supports 16ths with inches.
True, no decimal inches option. Well there is metric, but if that's a no-go;
One workaround would be to go into Edit > Preferences > Guides, Grids and Slices and set grid spacing 1 inch with 10 subdivisions. Then in the View menu, Show > Grid or the Ctrl + ' (Ctrl apostrophe) shortcut that will toggle the grid on and off.
This question goes back to CS3...maybe time for a feature request. Odd enough, Inches Decimal is an option in InDesign's rulers.