I too am having a similar problem:
Environment: RHEL 5 / X86_64 / Java 6 / Eclipse 3.4.2 JEE
Error:
Code:
An error occurred while collecting items to be installed
Problems downloading artifact: osgi.bundle,org.eclipse.emf.cdo.common,1.0.0.v200806180411.
Exception connecting to http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.common_1.0.0.v200806180411.jar.
http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.common_1.0.0.v200806180411.jar
Exception connecting to http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.common_1.0.0.v200806180411.jar.
http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.common_1.0.0.v200806180411.jar
Problems downloading artifact: osgi.bundle,org.eclipse.emf.cdo.server,1.0.0.v200806180411.
Exception connecting to http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.server_1.0.0.v200806180411.jar.
http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.server_1.0.0.v200806180411.jar
Exception connecting to http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.server_1.0.0.v200806180411.jar.
http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.emf.cdo.server_1.0.0.v200806180411.jar
Error closing the output stream for org.eclipse.emf.cdo.server.hibernate.libraries/osgi.bundle/1.0.0.v200806180411 on repository file:/usr/local/eclipse/.
Error unzipping /tmp/org.eclipse.emf.cdo.server.hibernate.libraries_1.0.0.v2008061804113938668312272400629.jar: Invalid zip file format
Problems downloading artifact: osgi.bundle,org.eclipse.net4j,1.0.0.v200806180305.
Exception connecting to http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.net4j_1.0.0.v200806180305.jar.
http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.net4j_1.0.0.v200806180305.jar
Exception connecting to http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.net4j_1.0.0.v200806180305.jar.
http://eclipse.ulak.net.tr/eclipseMirror/releases/ganymede/plugins/org.eclipse.net4j_1.0.0.v200806180305.jar
Error closing the output stream for org.eclipse.net4j.jms.api/osgi.bundle/1.0.0.v200806180305 on repository file:/usr/local/eclipse/.
Error unzipping /tmp/org.eclipse.net4j.jms.api_1.0.0.v2008061803053743619469237375528.jar: Invalid zip file format
Problems downloading artifact: osgi.bundle,org.eclipse.net4j.util,1.0.0.v200806180305.
Exception connecting to http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.net4j.util_1.0.0.v200806180305.jar.
http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.net4j.util_1.0.0.v200806180305.jar
Exception connecting to http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.net4j.util_1.0.0.v200806180305.jar.
http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.net4j.util_1.0.0.v200806180305.jar
Error closing the output stream for org.eclipse.tptp.platform.agentcontroller/osgi.bundle/4.2.201.v200808290100 on repository file:/usr/local/eclipse/.
Error unzipping /tmp/org.eclipse.tptp.platform.agentcontroller_4.2.201.v2008082901004872700835838943547.jar: Invalid zip file format
Problems downloading artifact: osgi.bundle,org.eclipse.tptp.platform.iac.administrator,4.4.200.v200809150936.
Exception connecting to http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.tptp.platform.iac.administrator_4.4.200.v200809150936.jar.
http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.tptp.platform.iac.administrator_4.4.200.v200809150936.jar
Exception connecting to http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.tptp.platform.iac.administrator_4.4.200.v200809150936.jar.
http://download.eclipse.org/releases/ganymede/plugins/org.eclipse.tptp.platform.iac.administrator_4.4.200.v200809150936.jar
Update: My problem seems to be fixed now. I didn't change anything so it was probably just a network/update-site glitch.