Vacuum questions

Question on vacuuming.

When you do a vacuum <table>, are the "freed" tuples available only the
table, or to the entire db, or to the entire cluster?

The reason I'm asking is that we are getting ready to preform a major
upgrade to our application that involves adding some new fields to
almost every table and populating them.  This is causing our test
conversion db's to double in size (we are assuming that every update is
causing a delete and re-insert behind the scenes due to the populating
of the new columns).  Anyway, we are working on trying to find the
fastest way to recover the space.

We have also had one test occurrence where after the upgrade, a vacuum
full would not recover the space.  However, when we bounced the
postmaster, and then performed a vacuum full, the space was recovered.
Any ideas on what might cause this?  This happened on a newly restored
db that was converted and then vacuumed full.  There would have been no
connections to the db after the conversion.

PG 7.3.4
RH 2.1

Thanks,

Chris