Some good news from the developers list today. The ECQL documentation is finished and the implementation is now available on trunk.
The OGC Common Query Language is defined as part of the catalog specification and provides a very readable text format for defining Filters. Filters are also use by the WFS specification to select data.
However the CQL format leaves out a few of the capabilies that WFS supports. The ECQL syntax documented above extends the CQL syntax allows you to do everything WFS is capable of.