Matt MacLean has just released an alpha version of asSQL - an ActionScript 3 MySQL driver. The SWC allows you to connect directly to a MySQL database from AS3, removing the need for a middle layer such as remoting or XML. As Mike Chambers points out, this could be extremely useful for Apollo applications which won't be able to use PHP locally to connect to a database.

I should also mention that Asql is available for download, and it looks to be a little further along in the development process. Hopefully these two get together and decide to combine their efforts.