Sometimes your first choice isn't always the right one - you learn through experience... and indeed I've talked about our wiki challenges before..

Our first choice for our internal wiki was mediawiki. For several reasons, it's stable, proven and continues to evolve and we could implement quickly without worry of license restrictions.
We've found over time though that even though there are lots of extensions to mediawiki they often do not meet what we need in an Enterprise environment - a robust tagging infrastructure, Enterprise Level access control with fine grained granularity, wysiwyg editing or that  the plugins are not portable across mediawiki releases as the codebase continues to evolve.

You may have already seen wiki.sun.com  is coming. It's currently still in Beta and will not open to the world at large for several weeks but we're hoping to leverage the fact that our DotCom group decided to go with  Atlassian's Confluence and possibly move to confluence.

Confluence has more of the pieces we need for an "Enterprise wiki" -a nice plugin architecture (with a good list of existing plugins) better editing, fine grained access control etc. plus it was good to find yesterday (amongst others) the Universal Wiki Converter  so hopefully we will be able to move most of our existing content into confluence without too much hassle.   I think it's time for some scalability testing and to double check the UWC works correctly ...  maybe play with some plugins too.