Mediawiki on mydomain.com without typing mydomain/wiki
 

I have tried alot of different things, including setting documentroot
to /var/www/mediawiki , which results in 403 error.

If I type mydomain/mediawiki, all is good. I want the wiki to be the default.. any ideas ??

set up a redirect in your web server (which you didn't mention) from / to /mediawiki

Forgive my ignorance, how ?

I'm going to assume you're using Apache2 because you still didn't say. Googled link: http://www.webmasterworld.com/forum92/82.htm

I didn't tell and you are right, apache2. Thanks for the brilliant link!