I've just finished moving the last of the content from GreyMatter and TikiWiki over to Drupal. I'll be changing the design of the whole site to celebrate soon. Of course, Drupal's been in use here for quite a while, but some of the articles I wrote back in 2003 seem to be getting a lot of attention recently, so I wanted to make sure I didn't break them!
If you're interested, I've put a bundle of the bits and bobs I used to do the conversions. It's all gnarly hacked up code, but if you're thinking of doing the same thing, it might be useful. I can't say as I'm in any position to help much if you want to do a job like this. I don't think there's any way you could make this a smooth process. Mine was hugely labour intensive as I did all sorts of fixes manually after the main transfer of data had happened.
I can impart a few words of wisdom though. Wikis all use different WikiSyntax, so it makes moving from one to the next a pain in the neck. For blog pages (which can have Wiki markup in them if you're using Tiki) I ended up fetching HTML from the server. Moving the actual Wiki was a bit of a hack, really consisting of changing a few frequently used Wiki tags on the fly. It still took a bit of manual messing around (but in my case, the Wiki wasn't very big).
Oh, and once you're finished, click the "reindex site" in the search module's settings page - if you don't, it says everything's up to date, but it doesn't index anything with a last modified date before today. In other words, all those old blog posts won't get indexed.
If you're doing this, make sure you've got plenty of spare time to hand. Good luck!