GeoServer requires special configuration to run in WebLogic out of the box:

• In the Weblogic console, set "Enforce Strict URL pattern" to false
• If you're running GeoServer older than 1.5.1 and want the make map previews work:
  1. unpack geoserver.war and deploy as a folder
  2. make sure the "preview" directory inside the GeoServer is writable
  3. add the followin section to GeoServer web.xml file:

     <mime-mapping>
        <extension>xml</extension>
        <mime-type>text/xml</mime-type>
     </mime-mapping>

We'd like to thank Luca Morandini and Jason Taixing for providing these informations.