Very interesting implementation of SQL injection protection; it can be especially actual for hosting providers considering the fact that they can't influence the code of web apps in the way of better security, but with the help of such mysql proxies it is possible to force hosting users to use alternate MySQL port that is listened by mysql proxy and redirects to MySQL itself. There is the analog of GreenSQL that is called MySQL Proxy. Its function is an analysis of the queries that are transmited between MySQL and Proxy for the purpose of performance. Stefan Esser created a plugin (on LUA btw) for MySQL Proxy that implements SQL Injection test as well. Check it here
http://www.suspekt.org/2008/08/05/mysql-proxy-heuristic-sql-injection-detection/