Do you explictly include the Ant jars into the project classpath ?
It seems that the Ant launcher of Eclipse is disturbed by a Ant jar included in the classpath.
I do not include the Ant jars into my projet classpath, but I added some optional jars (optional.jar & netComponents.jar).
I also noticed that I have the 1.5.2 version of Ant in my Eclipse, and that the actual Eclipse 2.1.1 is package with the 1.5.3 one.
Gonna check all this, and if I find a solution, I let you know.
At the end, I think that it's the optional jars that are the reason of this. If I don't put them, the packaging is working.
Also, I went with Eclipse 2.1.1 with Ant 1.5.3 in it...
I don't know exactly what was wrong, but it works like this, so...