Well, I just found this out: if I log out and back in, the additional three swap files are gone, and I'm left with the two original swap files:
% ll /var/vm
total 312512
drwxr-xr-x 4 root wheel 92 Sep 30 10:43 .
drwxr-xr-x 16 root wheel 500 Sep 24 00:50 ..
-rw------T 1 root wheel 80000000 Sep 24 18:03 swapfile0
-rw------T 1 root wheel 80000000 Sep 24 18:11 swapfile1
The other 240MB has been released:
% df -k
Filesystem 1K-blocks Used Avail Capacity Mounted on
/dev/disk0s8 1048440 778060 270380 74% /
Hurray!
So, it appears you can free up large amounts of accumulated swap by logging out and back in. I'm glad you don't have to restart to do it.
It seems OS X partitions swap into 80MB chunks, adding another 80MB chunk when you need more swap due to launching more apps. Usually in UNIX, the administrator allocates one big swap file of a specified size, and that's the hard limit. I suppose this is a better way as it somewhat grows dynamically (at 80MB increments), but cannot shrink.
I found that logging out and back in (I was logged in for 5 days) does seem to clear out memory, release swap, and generally make things run smoother. It does take quite a while to log out and in though (on my beige G3).
Learning more each day!