Problem with NAnt build (can't get filterchain to work)

I'm trying setting up a build-script in Nant (and I'm somewhat of a "NAnt-noob").
In this script I want to copy my entire codebase to a temporary directory, in which I later compile the code. When doing this copy, I want to replace certain keywords in the sourcecode, and for that I'm using "filterchain"s.
The copy works fine, but the filterchain doesn't seem to be executed at all.

Does anyone have any pointers?

I have included the content of my attempts so far in this post. This is of course not my real build, just a mock-up I made to try and isolate the problem (also downloadable from http://openspace.subsero.dk/temp/nant_problem.zip).

The setup is:
---------------
NAnt 0.84 (Build 0.84.1455.0; net-1.0.win32; release; 26-12-2003)

The buildfile:
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<?xml version="1.0"?>
<project name="Deploysource" default="deploysource" basedir=".">
            <property name="ApplicationName" value="MyApplication" />


            <target name="deploysource">
                        <copy file="source/testfile.cs" tofile="target/testfile.cs" inputencoding="utf-8" outputencoding="utf-8">
                                    <filterchain>
                                          <replacestring from="String" to="System.String" ignorecase="true" />
                                          <replacestring from="appName" to="${ApplicationName}" ignorecase="true" />
                                    </filterchain>
                        </copy>
            </target>
</project>


The source file:
---------------
//you are looking at appName

protected String testString = "hello";