I also signed up for this license plate when first came out, almost 4 years ago. I had a similar experience, but I sent in a check. It had been so long that the check expired and they asked me to send out another.
I was told over the summer that they were expecting to have 1500 sign ups soon and that I should expect to hear from the DMV this winter.
I'm not holding my breath, but it has been a long time coming.