Hello there,
I'm starting to project a CMS, but it's a little bit different, my theory is to build:

Centralized cms system, with single administration panel, all customers sign in to that panel, and see the data relevant to their site.
The cms should have a super-admin login (for me) with built in function for create new site easily (basically it just creates new web root and points apache/nginx to it), and to assign required modules/plugins for that customer.

Any customer site will have it's own database with their data in it, and a 'main' cms database. Approximate directory structure:

/www/
/mainadmin/
/customer1/
/customer2/
...

I have a dedicated server for development, just need to choose what *nix version to use.

As far as i think, the pros:
1) One centralized cms, any bug fixes affect all customers so new version of cms deploys to customers instantly
2) Easy managing of sites

And cons:
1) Scalability - what if need a second server? Should i copy that main cms there also - but from that minute, it's not centralized anymore (you solutions?)

Do you see anymore pros and cons?