I have uploaded an extension based on GeoWebCache 0.8.1 for GeoServer 1.6.3 to SourceForge [1]
The zip-file (geoserver-1.6.0-gwc-plugin.zip) contains a readme with basic instructions. Note that GeoWebCache (or any system that caches tiles) comes with certain restrictions compared to a normal WMS service:
Supported reference systems: EPSG:900913, EPSG:4326
Supported formats: image/png, image/png8, image/jpeg, image/gif
Tiles origin: -180,-90 for EPSG:4326, 20037508.34,-20037508.34 for EPSG:900913
Tile size: 256x256 pixels
Zoomlevels: Every tile corresponds to (world bounds / 2^n) where n is a positive integer.
You can read more about this at OSGeo WMS Tiling Client Recommendation [2]. If you have GEOSERVER_DATA_DIR defined GWC will store its cache in a subdirectory named 'gwc', otherwise it uses the temp directory of the container.
-Arne
ps. If you're already using the standalone version of GeoWebCache you should probably stick with that. The extension attempts to do some autoconfiguration, but does not allow you to configure layers manually.
1: http://sourceforge.net/project/showfiles.php?group_id=25086&package_id=129885
2: http://wiki.osgeo.org/wiki/WMS_Tiling_Client_Recommendation