Missing the point of autovacuum

Greetings,

I've been running Postgresql for many years now and have been more than
happy with its performance and stability. One of those things I've never
quite understood was vacuuming. So I've been running 8.1.4 for a while
and enabled 'autovacuum' when I first insalled 8.1.4 ... So in my mind, my
database should stay 'clean'...

As the months have gone by, I notice many of my tables having *lots* of
unused item pointers. For example,

There were 31046438 unused item pointers.
Total free space (including removable row versions) is 4894537260 bytes.
580240 pages are or will become empty, including 7 at the end of the
table.
623736 pages containing 4893544876 free bytes are potential move
destinations.

Perhaps I shouldn't be concerned with this? In all, I've got around 400
GB of data on postgresql, but am not sure how much of it is old data.
Many of my tables have 100,000s of updates per day.

Do I need to be running old fashioned 'vacuumdb' routinely as well? I
guess I just don't understand why autovacuum is not automatically doing
this for me and I have tables with so many unused item pointers.

thanks!
daryl