Details are here:
The thin client is pretty interesting as it can be used with non-IBM JDK’s and outside the Websphere environment. Also, don’t forget to poke around the samples in xmlfepsamples.ear.
If you want to try this out just get the free Websphere for developers:
and install the XML Feature Pack using instructions here:
Installing and uninstalling the XML feature pack on distributed operating systems
UPDATED : (11:30 AM EST) A colleague pointed out that customers should check their license for exact/specific scenarios that they are allowed to use the thin client for their production scenarios.