Hot on the heels of this announcement, Andi Gutmans has posted some of his thoughts on this new functionality and how it can help companies both large and small (he mentions specifically Yahoo! of course).
In my experience, databases have had a long history of being the typical bottleneck in PHP applications. There are many reasons for that.
His reasons include the fact that web apps being heavily database driven, PHP developers would rather write PHP than SQL, and that there's a lack of good, cheap tools to help find these bottlenecks. What he focuses on most, though, is the overhead caused by the need for multi-process environments to force each process to manage their own connections to the databse.
He talks about some stats on average connections and application speed, about efforts IBM has made to help the situation, and how it's not necessarily just the server's fault for delegating out the database resources - why can't they all adapt?