First, a much better solution, and then why I agree with the ACLU and the ALA on this one.
My solution is cheaper, doesn't require special programming, equipment, or a systems administrator. How about having the computer that is used for internet access be in public view? That connection is funded with public money, so shouldn't the public be able to see what the connection is being used for? Shouldn't have much of a problem with folks looking up porn on the public dole, then.
Now for the longer part. I remember this one from the e-mail newsletter I get. The American Libraries Association filed with the ACLU because they didn't want to be forced to install the software. Forced being the key word, but leave it to the individual library to decide on its own.
I work with computers for a living. I'm Microsoft certified to administer networks, and I have a degree in Computer Info Systems. I have to keep up with networking technology to stay qualified to work in my field, and that includes filtering technology.
Basically there are two main ways to filter content, and neither work particularly well. One is keyword, which scans the page for words that are on the "naughty" list. It's tough to make a computer program that thinks though, and it can't distinguish between some one's pet cat and porno pictures, if you get my drift. It also has trouble with fashion pages, and most older ones will block MSN's entertainment page.
The second is blocking particular web sites, usually via IP address. The problem with this is it only blocks known web sites, and new ones pop up all the time. It doesn't require any special programs (although they are available to automate the process with several known sites) to block a certain IP, and can be done at the gateway router or at the proxy server that all internet connections in the building go through. That's handy with porn sites that mimic actually educational sites. There is a world of difference in www.whitehouse.gov
Most "out-of-the-box" filtering software has an extremely high error rate, sometimes up to 80%. Server logs have to be reviewed by a systems admin and the program has to be tweaked to be effective, and then it is still pretty limited, although you can unblock sites that turn out to not be porn.
The problem is time. The logs get big, and administrators make pretty good money. Most companies don't want to pay their admin to do this, and most public agencies would be better off spending the money elsewhere.