"One of my favourite points to make is that a 'wiki' per se is not an application, it's a feature. A wiki by itself is a useful tool, but you can magnify it's use greatly in combination with other features (for example social features, blogs, comments etc)."
UPDATE: Matt also responds, also with a good comment:
"Wikis are essentially about simplicity and openness. So RS's comment that you'll find traces of wikiness (to a greater or lesser extent) on intranets going forward is right, I reckon. And lots of things will get badged wikis. But are they simple? And can I edit them? Oh I have to master CSS? There's an approval process? Ahem."