What version of Solr are you using?
I just tried this with the latest 1.4-dev version, and it works fine.
http://localhost:8983/solr/select?q=*:*&facet=true&facet.field=cat&facet.sort=true
Note that facet.sort=true/false has been deprecated in Solr 1.4
http://wiki.apache.org/solr/SimpleFacetParame
We have &facet.sort=true however our hierarchical fields are being
sorted alphabetically instead of by count.
Here is the field type and field definitions:
omitNorms="true" positionIncrementGap="0" indexed="true"
stored="false" delimiter="/"/>
multiValued="true"/>
Any help is much appreciat