Was part of group discussion on another board where the subject was "truly alien." I then remembered an interview in Unbound Worlds with author China Mieville, a way over-educated young Brit who writes brilliant, weird SFF. Rather than explain it, I'll let the question and answer speak for itself.
Portraying authentically alien intelligences- in the sense that they’re not just humans with pointy ears or green skin – must be an incredibly difficult task for an author. How do you approach this?
I don’t just think it’s difficult, I think it’s literally impossible. If you are a writer who happens to be a human, I think it’s definitionally beyond your ken to describe something truly inhuman, psychologically, something alien. However, that doesn’t mean there’s no point trying, or that you can’t do some pretty interesting things with flaws and failures, and the very asymptotic aspiration is pretty exciting. You can play games – you can imply consciousness beyond ours, you can hint at things obliquely, you can not say too much, you can have a character describing in passing. I don’t think you can succeed, but I think you might just fail pretty wonderfully.
Though there are some honourable Forehead-of-the-Week aliens, too, in our tradition. SF seems to oscillate between cheerfully unalien aliens, and inadequately but, crossed-fingers, interestingly more alien-ish aliens.
Shameless plug: Mieville's unbelievably great urban fantasy police procedural, The City and the City, is in production as a 4-part BBC series. Highly recommended book and if BBC 2 doesn't totally muck it up, the show will be extremely compelling.