Unable to load plugins from behind proxy server

My company is behind a firewall.  I have the proxy server settings configured in IntelliJ.  They work in Eclipse, so I'm sure they are correct.  However, when I try to update plugins, I get the following error (from idea.log):

2011-09-20 09:29:30,220 [  70663]   INFO - .ide.plugins.PluginManagerMain - Server returned HTTP response code: 400 for URL: http://plugins.intellij.net
java.io.IOException: Server returned HTTP response code: 400 for URL: http://plugins.intellij.net
 at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1441)
 at com.intellij.util.net.HttpConfigurable.prepareURL(HttpConfigurable.java:147)
 at com.intellij.ide.plugins.RepositoryHelper.process(RepositoryHelper.java:62)
 at com.intellij.ide.plugins.PluginManagerMain$10.construct(PluginManagerMain.java:338)
 at com.intellij.util.concurrency.SwingWorker$2.run(SwingWorker.java:155)
 at java.lang.Thread.run(Thread.java:662)