This particular feature can't be done purely in the toolkit for all cases, though we can make it easier for apps to add support for it, and can probably do it for simple cases. The keynav isn't finished, e.g. in gtk-demo the tree widget has no keynav. But it is definitely much better. Try the color selector in gtk-demo for example. App developers can set the tab order for any container.
Underline shortcuts are now easy to install and used throughout gtk-demo.
Numeric keypad support is already better in 1.3.5 and complete on my local disk. Something like the edit key binding dialog in Xchat. It is something that should be in most application. If it is not put into gtklib, then everybody will start to write their own version of keybinding dialog. Well I really, really hope you guys get to finish good keynav support for 2.0.