Thanks for your replies, ponyboy and Jay.

There are currently five or six blog domains sitting around that I want to fill with life. They all cover different niches and I want to use them as single domains, with the option to build micro niche blogs within those domains by using sub domains. So I will definitely make use of sub domains.

My server admin seems to have found a way to achieve the goal to work from one WP installation with one login/pw. I've briefly read his email explaining how it should work. Now I just have to see if it really works the way I want it working. I'll keep you updated with my progress once I figured out everything.