One important thing to note about Expression Encoder is that it can be used to build the Silverlight player experience around an already encoded WMV file, so it can be used in conjunction with other compression tools.
Expression Encoder is based on the Windows Media Format SDK 11, so all the registry key tweaks I've been discussing on this blog work perfectly with it.
(repost from http://on10.net/Blogs/benwagg/silverlight-expression-encoder-10-released/ )