I don't remember the particular document I got these plans from but it was in relation to SSH regional plan, probably a special transportation annex to the SSH plan. I was pleasantly surprised to see the plan being approved unanimously by the seven municipal authorities that form the SSH. This general outline seems to have support in different municipalities and across party lines so I guess it will move forward in some form. I consider BRT more likely than light rail though, at least to begin with.Where did you get these plans? It´s probably a very interesting read.
In June/July of this year, Samtök Steitarfélagana á Höfuðborgarsvæðinu (the collaberation between the councils in the capital area) released the new city plans from 2015. They outline how the city should grow, and shows development areas. The Mayor of Reykjavík has signed under this plan, among other officials.
ssh.is/svaedisskipulag - In icelandic
In this report, they outline a light rail system, called ´Borgarlína´, which seems to be the same system that Bjarki posted. This means that light rail transport is ´on the agenda´ for Reykjavik, but preemptive studies are still in the works. It will be interesting to see where this will head.