The basic argument over Quinoa is regarding Kitnyot - does Quinoa enter the kitnyot ban (an Ashkenazic problem only). No one doubts that Quinoa is a grain - however it is a grain that was not known at the time when Kitnyot was banned by rabbis in Europe - and hence there is a strong argument that the ban shouldn't be included to encompass new grains. Additionally certain attributes of Quinoa make it less likely that it would have been mixed with other grains - the conceived basis for the Issur of Kitnyot. Those who want to follow the full halachic discussion can read a good summary here.
The OU 2011 Pesach guide has a whole page dedicated to the question of Quinoa and Passover - but after only a very very brief outline of the problem, ends with this dissapointing line:
The OU has not taken a position about the use of quinoa on Passover and believes
that this decision should be made locally. The information in this article is intended to
enable an informed and enlightened conversation.
Weirdly enough right after the OU states that it does not have a position on Quinoa, it lists it at the bottom of the same page in a list of kitnyot. That list includes potatoes as well (as noticed by Rationalist Judaism). Putting the two together it would seem that the OU thinks Quinoa is kitnyot - but a type of kitnyot that possibly can be eaten during Pesach.
As a public service here are some of the more prominent Israeli poskim, and their cities, to help you make your "decision locally". I hope someone else makes a Quinoa map of Israel, and does a similar list for America.
Quinoa is delicious and good to eat!
- Harav Avigdor Nevtznel - Jerusalem (old city) - he is quoted in this article.
- Harav Dov Lior - Qiryat Arba.
- Harav Sherlo - Rosh Yeshivat Petach Tikva. (sidenote: This must be a unique list where Harav Dov Lior and Harav Sherlo agree with one another!)
- Harav Avraham Yosef - Holon.
- Rami Berchyaho - Telmon.
Quinoa is a Kitnyot:
- Harav Ariel - The rav of Ramat Gan. - Quinoa is similar to Corn, and hence kitnyot.
- Harav Noibert - As quoted here.
- Harav Elyashiv - The whole world if you are Haredi, otherwise Bnei-Brak. Quoted in the connected article.
- Harav David Lou - Modin
- Alon Shevut - An answer given in the name of anonymous rabbi from the gush yeshivah. I'll point out that the same Rabbi who answered that Kinoa was Kitnyot said the opposite here - so I'm going to call this one inconclusive.