"com.salesforce.ide.api.metadata.types.Metadata$JaxbAccessorF_fullName cannot be cast to com.sun.xml.bind.v2.runtime.reflect.Accessor"

Saw that on my newly installed Juno when I tried to refresh Force.com projects (or pretty much everything else after that).  There's some discussion with no definitive answer (various workarounds work for various users).  For me it's fixed by using JVM 1.6 instead of 1.7 (update 21).