I've Mediawiki install with large existing userbase, but i wish to integrate it seamless in new Wordpress blog. If you'll be able to integrate MediaWiki with Wordpress using existing Mediawiki's user database ?

Okay, I've not attempted this, because I wish to integrate the other way round, only one means to fix your condition may be to set up the OpenID MediaWiki extension:

http://www.mediawiki.org/wiki/Extension:OpenID

Then in Wordpress install the OpenID extension and configure it to simply allow OpenID sign ups in the link to your MediaWiki install.

I'll most likely be utilizing the same extensions, but configure them to be used the other way round, because the other integration options pointed out above predict date and hard to keep.