I have just starting using a trial copy of 3.0.4 (Build 701) and have had a world of trouble with my existing build script. Although the script works perfectly well at the command prompt (win2k sp3) it did not work in IDEA.
First it was the ]]> task. I ended up putting all the xdoclet jars into the /ant/lib dir. Then it was junit and the dreaded "Could not create task or type of type: junit." problem. Are these an IDEA problems? Not really, but IDEA isn't automatically grabbing the files in the /ant/lib dir.
Do yourself a favor. Open up your build script in IDEA, get the Ant Build Properties screen up and add every jar in the ant lib directory to the additional classpath tab there. That's as close to a silver bullet as you are going to get.