Just added a new subscriber node to my slony replication instance.....
while all the data is now synched, I was surprised to find my subscriber 
database utilizing significantly less space than the master node.

In researching (I just assumed I had forgotten to subscribe a specific 
set or had neglected a particular table) I found that the tables 
themselves were nearly identical in size.....but the indexes in some 
cases were significantly different (in one case, the master node disk 
size for index for a specific table was 6 gigabytes...on the subscriber 
node, it was only 1 gig).

I would have understood if the tables themselves were smaller (dead 
tuples in the master node causing bloat)......but in the indexes?
I run autovacuum on the master node...and I assume it is doing it's 
job......

I am assuming that  empty index entries are not reused and that only a 
re-index on the indexes (on the master node) will recreate (and 
shrink)....but this requires a lock on the entire index, correct?

These are all btree indexes (under postgresql v 8.1X).