Naka-dashi (㊥出し) started as fetish site, mostly bondage. It was so feisty that I did not dare to link to it, and the banners they ran would never be seen at Sankaku outside of 3 layers of warnings. But a couple of months ago they suddenly mellowed out. It turned out that my observations were not just my delusions. Yesterday, Bj0rn, the site proprietor, mentioned at #animeblogger that he entered some media deals that required him to tone it down. And today, the site was rebranded as NK-DS. See, the weird character "中-in-a-circle" with "出し" in the old brand were too suggestive of penetration and ejaculation... It's the weirdest PR story I've heard in a while. Also, one of the oddest characters in Japanese fonts. I wonder what it was originally for... Surely it was not introduced into the national standards specifically to mean "porn". Although, with Japanese you never know!

UPDATE: By e-mail from J.Greely:

The complete set of enclosed kanji in Unicode (3280-32B0) is short and looks like basic concepts used in commerce. The Mac IME lists only a few of them as present in the JIS character set, and classifies them as abbreviations: ㊤ ㊥ ㊦ ㊧ ㊨. Most of the time when you see circled kanji, though, they're display fonts or synthesized characters; I haven't found anything that uses the actual symbols.

The Japanese term to search for, by the way, is 囲み文字. You'll find lots of how-to pages on how to make them in Word, Excel, and OpenOffice. :-)