Speaking of databases and read/write splitting, Oracle had a session at OpenWorld about it.

Building a read pool of database replicas isn't something I usually think of doing with Oracle, mainly due to their non-zero license fees.  It changes the scaling equation.

Still, if you are on Oracle and the fees work for you, consider Active Data Guard.   Some key facts from the slides:

• Average latency for replication was 1 second
• The maximum latency spike they observed was 10 seconds.
• A node can take itself offline if it detects excessive latency.
• You can use DBLinks to allow applications to think they're writing to a read node.  The node will transparently pass the writes through to the master.
• This can be done without any tricky JDBC proxies or load-balancing drivers, just the normal Oracle JDBC driver with the bugs we all know and love.
• Active Data Guard requires Oracle 11g.