Since 6u10 we've apparently signed all the 32 bit JRE .exes (see bug 6688899).
However :
1) install/make/Makefile skips the "sign" subdirectory for 64 bit apps.
I can't think of any valid reason to do this, except for being a hold
over from before we had a proper 64 bit installer and it wasn't thought
necessary.
2) The "sign" subdirectory as far as I can tell looks at JRE_IMAGE_DIR
which is used only to create the JRE installer.
This apparently means that all binaries installed by the JDK installer
(both those in jdk/bin and jdk/jre/bin) are unsigned.
This is a very confusing situation.