ANT build fails with NullPointerException on the <input> tag.
This error drove me crazy and took me longer than I would like to admit to figure out - the solution was to run the ANT script "in the same JRE as the workspace". There is an eclipse open Bug 281837 from 2009 but without any follow up so perhaps do add your comment/vote there to create some traction.
To change the JRE setting right click on the ant script and choose "Run As > Ant Build... > JRE tab".
If this has helped feel free to follow me on twitter @danielsokolows.