Every update of dev-java/icedtea-bin should trigger rebuild of dev-java/icedtea-web. Otherwise I need to rebuild it by my self (emerge -1 dev-java/icedtea-web).
This is already fixed in 1.6.1. We can't make it stable or clear the older versions just yet for reasons that are hard to explain but we're working on it. *** This bug has been marked as a duplicate of bug 538144 ***