I am running two sites on 3.0.5 and using symbolic links. The dev site database settings are ignored in mysite/_config.php and uses the production database settings. I know this because when I ran a dev/build on the dev site, the page loaded with no errors but displayed the production database name at the top.
www.domain.com and dev.domain.com
In the mysite/_config.php,
each site has a unique username, database name and password.
db name: prod_db
db name: dev_db
I added 2 new PHP files mysite/code on dev.domain.com and I ran dev/build on dev.domain.com, the page loaded with no errors and the message at the top says "Building database prod_db using MySQL 5.1.56-log."
I logged into www.domain.com/admin/ and added a page to the CMS and then logged into dev.domain.com/admin/ and the new page appears there as well.
The 2 new PHP files do not exist on mysite/code on www.domain.com yet when I review the dev/build results from the prod_db, the new tables are listed that were created by the 2 new PHP files from the dev site.
I deleted the silverstripe-cache folders on both sites and recreated.