Auto Update SP2 Download Behavior

I'm a little confused about how BITS and the Windows
Update site will make use of bandwidth when SP2 is
downloaded by Automatic Update.

I thought I understood that bandwidth will be detected on
the particular network and a small amount of it will be
used evenly over each machine, that amount depending on
how much is available overall.

However, I just read that it's more like a few machines
on the network at a time will get it fairly quickly until
all machines have downloaded it. There's a huge
difference between the two when you are administering a
large network of XP machines. In the second scenario, it
will be even harder to keep track of which machines have
downloaded it.

I ask this because today I saw something that may suggest
that the second scenario is correct. I have looked at
the Windows Update icon in the Notification Area on
several machines and it has sat at 0% for over a week.
Yet last night I pulled a machine out of a closet that
hadn't been on the network at all in the last few weeks,
put it on the network, and by this morning it had
downloaded the SP.

Can someone clear up this confusion, or even better point
me to a KB article that explains it all?

Thanks