I've seen some webpages that describe the SchemaExport Ant task. But it did not work when I tried to use it. The documentation for it was sparse. In any case, I traced through the underlying Hibernate code and found out that you can generate the schema creation SQL with just three lines of code:

package com.codebits;

import java.io.File;

import org.hibernate.cfg.Configuration;
import org.hibernate.dialect.PostgreSQLDialect;

public class Play {

 public static void main(String[] args) {
  Configuration cfg = new Configuration();
  cfg.addDirectory(new File("config"));
  String[] lines = cfg.generateSchemaCreationScript(new PostgreSQLDialect());

  for (int i = 0; i < lines.length; i++) {
   System.out.println(lines[i] + ";");
  }
 }

}

Place your .hbm.xml files into some directory (I called mine config) and then execute the above class. Your schema creation script will be displayed on the console.