How do I version a product of a build?

There is no single answer, but here are some thoughts:

• Use <propertyfile> and a properties file to keep an incrementing value
• Use the appropriate source code management tasks (i.e. <cvs>) to checkout and check back in the properties file used to track version numbers
• Load the properties file in using <property file="..."/> and use the version property for the appropriate directory names, file names, and manifest entries.
• Do not name JAR files with the version number in its name if you can avoid it - name the distribution directory with the version number instead. It makes using your JAR file in automated environments much nicer to deal with a constant name for the JAR file when upgrading.