Hi Bryan,
I was offered the opportunity by Avaya/Nortel to help test the code but the plate is just way to full right now.
I just took a quick look and don't see the software available on the Nortel support website although I could be wrong. I don't have any direct experience with 5.1.2.0 as of now but I know that Avaya/Nortel is very keen to avoid the nightmare that was 4.1.6.x and 4.1.7.x so they are being very cautious in their deployment.
I believe there are a few users (here on the discussion forum and following the blog) that have deployed 5.1.2.0 but I'm not sure they'll be able to openly comment due to potential non-disclosure agreements and whatnot.
Perhaps someone else will be able to comment!