In web browsers you can press Ctrl+Plus and Ctrl+Minus (or use Ctrl+Mousewheel) to zoom in and out of a page. This is entirely transparent and works on any page. It's not simply scaling up the rendered image, but actually scales the width, height, text size, etc of everything in the page (which can cause the layout of elements to change, and may even trigger alternate stylesheets based on the lower reported window size).

GTK+ should support the same. It should be possible, in any GTK+ application, to zoom the app in and out using some universal shortcut key. This seems like it should be relatively trivial to implement - just multiply all widgets' dimensions, font sizes, borders etc by the zoom factor, and maybe change the reported window size to keep layouts correct. In fact if GTK+ supports varying DPI already, I'd think zooming would just be a matter of changing the DPI.

The use for this would be for accessibility (for people who have somewhat poor eyesight and need to zoom in a little, but not bad enough to justify permanently increasing the font size system-wide) and for a "10-foot UI" (I like to watch live video streams from the couch; some of these have an IRC channel, and I'd love to zoom in the IRC app so that I can read chat from the couch as well).

Maybe also interesting is the possibility to scale individual widgets. (Maybe Ctrl+Shift+Mousewheel over the desired widget?) e.g. you could scale up just the text field in a text editor (to give the same effect as changing the font size, but temporarily), or the one button you have to click often, while not having gigantic menus and controls.