I would like to launch a closed blog network site for a very specific
subject. I would like it to be a portal, where each user will have the
possibility to open its own Blog after a short registration parocess.
For implementing this, I'm looking for a Blog platform, which will
allow me, the admin, to manage Unlimited weblogs with Unlimited
authors with all regular Blog features.
Some platforms I already found are:
http://www.sixapart.com/movabletype/
http://www.lifetype.net/
http://www.reliableblog.com/
I would appreciate if someone can help me decide what is the best blog
platform or offer me other available providers,
Thanks,

You may want to specify what programming language you prefer.  Have
you tried hotscripts.com?

Thanks...
Well I was thinking about PHP and yes, I have checked hotscripts.com
but since I'm not a developer I preffer taking a complete solution,
which is widely known and used.

Have you checked out wordpress?
http://wordpress.org

Now, one thing, I'm not entirely sure what it takes to offer users to
register and eventually create their own wordpress blog, however,
there's a few sites that already do something similar to what you're
asking and use wordpress.

Wordpress's own blogs site
http://wordpress.com

Yahoo has recently partnered with wordpress to offer paid wordpress installations.
http://sbs.smallbusiness.yahoo.com/webhosting/problogs.php

I'm sure if you ask the support guys at the wordpress site, they'll be
able to help you set things up.
http://wordpress.org/support

Here's the plugins page
http://wordpress.org/extend/plugins

And the themes page...
http://wordpress.org/extend/themes

And below, an example of wordpress in action (my website)
http://www.justinpereira.com

If you say best blog platform I say ExpressionEngine
http://www.pmachine.com/ee/feature_grid/