Spent the better part of a day looking into how to include Eclipse's .classpath file into an Ant build.xml classpath w/o having to retype, copy it, or whatever; in other words, trying to keep Eclipse's build in synch with Ant's build. Ends up, the easiest way is to simply define properties to the paths, then include those in the path definition. Silly, really. There appear to be ant task add-ins [1,2] for Eclipse, but for what I was doing, they were overkill. I'll have to look into them later.

In order to get Eclipse's Ant build to work, I did have to do two things:
  1. <property name="build.compiler" value="org.eclipse.jdt.core.JDTCompilerAdapter"/>
  2. Have Eclipse's Run->Ant Build..., JRE run the same JRE as the workspace