Disk cache causing swap usage

I have 1 GB of RAM and a 2 GB swap partition.  Previously, I would 
almost never see swap used (or at least no more than 4k used) under 
normal use, but that appears to have changed recently.

I am running
FreeBSD 6.0-CURRENT #3: Tue Jun 14 14:25:20 PDT 2005
root_at_brian:/usr/obj/usr/src/sys/MYKERN

My testcase for this problem is to run
dd if=bigfile of=/dev/null
and watch the free space drop.  I expect that to happen, as the disk 
cache holds onto what has been read.  But when free memory runs out, it 
starts paging out to the swap file, when it should have plenty of old 
cache to throw out first before resorting to the swap file.  I'm pretty 
sure it didn't do this before, but I ran into the 20050609 issue in 
UPDATING when trying to run top on an old kernel to test it.

This effect is also observed during normal use, and makes the system 
sluggish eventually.  I found that setting vm.defer_swapspace_pageouts 
to 1 seems to prevent paging out, but I don't think it completely solves 
the problem, as during the dd test cursor movement still becomes erratic 
by the time free memory runs out.

Is this a new bug in the kernel?  Is anyone else seeing it?