I have added a new cvs task, <cvschangelog/> that produces an xml report of the changes to your cvs repository between a given start date and end date.  The code makes use of an extension to the sharpcvslib binary (a most excellent contribution provided by Gerald Evans) so please make sure that you are using the scvs.exe binary provided with NAnt before reporting bugs. 
Please consult the docs for specifics of use (hopefully available in the next nightly build) and if you encounter anything that requires clarification drop me a line.  Also just a final note, the xml file generation is also available via the commandline client (use scvs --help xml for details).