As far as I know, no, you can not longer write reviews from apps downloaded with promo codes. It seems that the change was recent (couple of months ago). A google search showed lots of articles about but I couldn't find an official Apple statement about it.
You still can. I've had friends review my apps even though I gave them all promo codes. And for the record, I don't think it's unethical to ask a few friends for a favor.
It'd actually recommend as a marketing strategy to hand out as many promo codes as possible. To bloggers, friends, anybody who asks or complains about your app on twitter. The goodwill you gain is worth far more than the potential lost sales. Even at $20, I think giving out promo codes has made me much more in good business than the thousand bucks in "lost sales."
