mysql cluster is slow - main gripe is horrendous query plans - any non-trivial queries take forever

left with mysql master/slave or master/master - main issues are how to load balance (replication lag an issue)

mysql proxy, sqlrelay are load balancers that make db scaling transparent to app - don't account for replication lag.  Since mysql proxy does force transactional queries to master, you could wrap sensitive reads in a transaction (e.g. view page after adding comment)

 There are a handful of application level load blanacers

http://github.com/technoweenie/masochism/tree/master/lib

http://blog.rapleaf.com/dev/?p=5

http://seamlessdbpool.rubyforge.org/

http://github.com/fiveruns/data_fabric/tree/master