i know its easy enough to put the database on a separate box and use caching tricks to get more mileage from my server. What i am looking for is information about how people are using multiple web server boxes all pointing to a single data source box. It can be cheaper to use multiple smaller instances at rackspace or amazon. I am guessing you designate one as the upload/write to database box. anyhow just looking for information in this area from people who are already doing it.
There are plenty of articles out there about it. "Scaling WordPress" or "Scale WordPress" in Google will turn up some of those. Example: http://wpmu.org/scaling-wordpress-wpmu-buddypress-like-edublogs/
For the database end of things, there are plugins existing:
http://wordpress.org/extend/plugins/hyperdb/
http://wordpress.org/extend/plugins/shardb/
sometimes just having the right key word to search on makes all the difference. n-tier wordpress or things like are not the right ones to use. thanks tim.