Sometime in the last few months someone managed to worm their way into our site and hack it up. Fortunately they didn't do anything too nasty, just added some (invisible) links to friendly porn sites at the tail end of some of our pages. Why invisible links? We have a pretty high page rank on Google, and so links from our site are valuable because they make the page rank of the target site higher.
Fortunately for us a couple of astute Unshelved readers (is there any other kind?) saw the problem and let us know (thank you!). I believe I have now purged all the nastiness, and our host and I are looking into how this may have happened. I'm also in the (slow) process of upgrading our site in such a way that this particular type of attack should be impossible in the future.
Meanwhile if your Internet filter was blocking our site, this is why. It should stop now. If you are still seeing problems please let us know!