PG 7.2 on Linux: where's the space?

We upgraded from 7.1 to 7.2 recently, and saw the used space on our
database partition drop from something like 80% to under 20% (as
measured by 'du').  Hurrah!  However, it's about four weeks later and
we're back to over 80% again.  Is the solution:

1) More frequent VACUUM FULL operations?

2) REINDEX our biggest tables?

3) Periodicly dump and reload our biggest tables?

I'm really hoping this last one is not the answer, as we'd have to
take the system offline for hours every week to accomplish this.