[puns]

Their new designers are adding a lot more visual touches to the interface, too. The original 37signals style consists mainly of HTML elements styled with CSS, and with all the controls unstyled, e.g. form controls. When you're not creating new images for buttons, backgrounds and boxes, then there really is little point to use Photoshop -- you just need to mock up the layout, and that can be done on paper.

When you're adding a lot of visuals, you'll end up using them in the final product, which is where designing that stuff and laying it out in Photoshop becomes more practical. It really depends on the style of the UI and how heavily it relies on custom images.