I think I've fixed this and, fingers crossed, not broken anything
It seems that the offending packages rubbyripper
do not appear in the F17 repos so on discovering this I checked if anything significant depended on them with:
rpm -q --whatrequires rubyripper
Being fairly satisfied that nothing important was using these packages (or the other F15 ruby packages listed by yum as being problems in the rpmdb) I simply removed the lot.
Nothing has exploded...so far...