Yeah, I did my tests too after my question and found the same thing. Back to work.No. Each Subdomains are treaten differently like a "new" domain name. Take an example of wordpress.com or blogger.com. If you open a new account (sub-domain) you will need to build backlinks to rank well. I test it with my own domain that have a 5 and half years and created a sub-domain and was not ranked, even indexed when I first started with that sub-domain.
Yes, that is what I found.So in case that I want to build auto blogs with Wordpress MU, I better go sub-dir not sub-domain?