This article is over a year old... has anyone heard anything more recent?
http://www.losslessaudioblog.com/2006/08/30/built-in-flac-support-coming-to-leopard/
I found this a while ago in another forum...
You may also notice that FLAC (Free Lossless Audio Codec) also becomes available in the Leopard preview... not sure any application in the Leopard preview is using it though. However, you can certainly access it via APIs or the command line tool, afconvert.