Phing not loading .properties file

I have the following build.xml and build.properties files:

build.xml

<?xml version="1.0" encoding="UTF-8"?>
<project name="CoolProducts" basedir="../"
         default="dev" description="CoolProducts front-end">
  <!-- set default config -->
  <property file="build/config/build.properties" />
  <target name="dev">
    <echo message="{$dir.css}" />
    <echo message="{$dir.build}" />
    <echo message="{$dir.source}" />
  </target>
</project>

build.properties

dir.source = .
dir.css = css
dir.build = build

I expect my output to be:

css
build
.

but it is:

{$dir.css} 
{$dir.build}
{$dir.source}

Do I have something configured incorrectly?

It's ${property}, and not {$property} that must be used to display a property.