Dropping indexes doesn't seem to release space to the OS. Can it?
The space should be freed as soon as the index is dropped.  Possible explanations for what you show above are:


The index is in a tablespace on a different filesystem from the one
you're checking.
The OS/filesystem has some lag in providing up-to-date free space
information.
Something else is eating the free space as soon as it becomes available.
Something is holding open index files; lsof might be useful here.


If you run this query, you can see a little information about the index, including the filesystem number and the number used for filenames making up the index.

select oid::regclass, reltuples, relpages, reltablespace, relfilenode
  from pg_class
  where oid = 'mcdata_2011_07_host_idx'::regclass;


Look for files with filenames based on relfilenode both before and after dropping the index.