Hi Ton!
There's a plugin already doing that, or something similar: WikiFormat (http://mt-plugins.org/archives/entry/wikiformat.php).
It takes your entry body and looks for wikiwords, building links to the wiki. I think it also does more advanced wiki formatting, but I'm not sure.
Hope to be of some help,
Bru

Hi Riccardo,

No that is something different, that is building links to the wiki if wikiwords are in the posting.
What I am trying to do is to be able to decide per blog entry if I want to crosspost it into the wiki. So it is about putting blog-entries in the wiki.

And then, if an entry is in the wiki, I want to add links added in the blog-entry to the wiki, to edit or see the wiki-page.

ok, got the picture now :)
The idea is really interesting, it adds a lot of "integration" feeling between wiki and blog (I got a wiki too, but they feel really like two separate things right now).

About your plugin, actually MT3 does the trick, letting you define callbacks where to put extra functionality in various step of the publishing process.
In your case, you could do something like:
MT::PluginWiki->add_callback ("pre_save", 1, $plugin, \&add_post_to_wiki());

To have the same effect on MT, I think you'll have to play with the perl API, and write an external agent that periodically (either as a demon or via cron) goes through your posts, looks for categories, pulls the post's body, and push it into the right wiki topic...