If you read kde-core-devel or kde-devel, you'll have already heard about the proposal which the HCI team has produced for KDE 4. With input from usability, accessibility and documentation folks, they've produced a detailed design document for an improved way of linking tooltips, whatsthis and documentation. The document is quite long, so here's a mini summary of some of the main points:
At the moment, tooltips, whatsthis help and documentation are generally produced and presented to the user quite separately. In the HCI team proposal, they're brought together in a sort of natural progression of increasing detail: single-word or -phrase tooltips -> single paragraph whatsthis -> detailed docs. So, tooltips will present a way of progressing to whatsthis, which will present a way of progressing to the full documentation. This is one element of the proposal...
...The second element is to improve the usability of the whatsthis windows themselves. At the moment, the whatsthis help appears in "bubbles" which disappear as soon as you click anywhere. That's obviously not ideal, so in the HCI team design, they're "upgraded" to full windows, so you can keep them open while following the instructions in them, have multiple whatsthis windows open, move the windows around, and so on.
So far, it's just a design - what we need next is an implementation, so it can be tested, refined, and most importantly, so we can start putting content into the tooltips and whatsthis once KDE 4 applications are ready for it. So, if you're comfortable coding with C++/Qt/KDE, and you're looking for a way to get involved with KDE 4, please do contact me, and I can put you in touch with the HCI team, who will work closely with you to help get an implementation off the ground.