Did someone notice that you get all subdomains while executing the site:www.domain.tld query on google? although only pages on www.domain.tld and no subdomains should be listed.

Example: http://www.google.com/search?q=site%3Awww.ebay.com

Yep, but it looks like a glitch, rather than a design decision :-

http://www.seroundtable.com/archives/007250.html

"January 22, 2007 Google Seeing Sub Domains As Part Of Main
Domain? Site Command Change
[...]
Update: Google has informed me that this site command
change was not an intentional change and we should expect
the old behavior to return relatively shortly."