2.5.1 install ignores table_prefix for multiple blog/single database

[ssinn]

​http://wordpress.org/support/topic/174688?replies=9
We are installing multiple instances or WP using a single database. table_prefix in wp-config.php is used to isolate each installation.
The primary installation of WP 2.5.1 works fine, using the table_prefix in wp-config.php. The second or later installation seems to ignore the table_prefix and returns the error "Already Installed" when accessing ​http://blog.domain2.tld/wp-admin/install.php.
The setup we are using worked correctly in 2.3.3. I suspect wp-db.php is doing something differently between versions.

[mrmist]

No movement.

This works from stock 2.5.1.