Slashdot looking for open proxies?

I saw the following somewhat-strange line in my web server logs today:

216.34.181.45 - - [25/Aug/2008:10:23:51 -0500] "GET http://tech.slashdot.org/ok.txt HTTP/1.0" 401 523 "-" "libwww-perl/5.812"

That web server is running on the IP of my home router. The requesting IP appears to be a Slashdot machine. My guess is that they are trying to find out who accesses their site through an open proxy. But why? Is there another reason they might send a request like that? Do they ban proxies if they find one?

UPDATE: Apparently, they do in fact ban open proxies (according to this). Supposedly a lot of comment spam comes from them. I wonder if it would help blogs at all to do something similar?