give more of the logs/stacktrace.
i had a similar problem today. if it is the ant.task.properties or something like that, you just need to retry. i don't know why it sometimes fails. all help with that is appreciated.
previously i thought that problem was because the jpdl library was not build before ant scans the classpath. with the new build scripts that problem should be fixed and i still got that error this morning... so i'm a bit clueless there.
This problem is fixed - the build target of the build/build.xml was calling into a nonexistant subproject build.xml. I was going to post a fix... but Tom got there ahead of me. Looks like he made a couple other fixes to.
It works now in any case.