The environment variable defined via the Version Number plugin (versionnumber) resolves to null by the svn-tag plugin.

Example:
1. "Create a formatted version number" is checked
2. "Environment Variable Name" is set to BUILD_REVISION
3. "Perform Subversion tagging on successful build" is checked
4. "Tag Base URL" is set to "tags/${env['BUILD_REVISION']}"
5. When doing a build "null" is used in place of "${env['BUILD_REVISION']}" but BUILD_REVISION is used and properly resolved by ant when used to define a property