Expression Encoder (formally called Expression Media Encoder) is now available to download.

http://www.microsoft.com/expression/products/download.aspx?key=encoder

If you've been using the earlier preview, then James has a great write up on his blog of the new features that have been added since that time.

http://www.clarkezone.net/default.aspx?id=c7a6bb98-f42b-49b6-aeba-e061722d69b8