It's in AHD4.

It's also in the OED2 (online). It's also in Merriam-Webster's Online Dictionary 4th (link). See Outlook's output (link). Goodness, it's even in Encarta. Basicially it's in every dictionary I use, except the last mentioned herein above.

But worst of all, who has even heard of this silly word?

Uh, I have. So, I've read a lot of Russian history. The first citation in the OED2 is from 1923. Long enough to be brought into the English family of words, word or otherwise. Another great word from Russian (although it ultimately comes from Latin) is intelligentsia. It's been in English slightly longer than starets, by a decade or two, that its pronunciation has changed from a hard g (/g/) as in the Russian to a soft one (/ʤ/) as is usually the case before an e.

Ceci n'est pas un seing.