Health Care for America Now (HCAN) does not support single-payer healthcare. They support Obama’s mixed public/private insurance plan.
It’s true that HCAN supports a choice of private or public plans. Which means they don’t support single-payer right away. That doesn’t mean that what they’re proposing won’t lead to single-payer eventually.
Look at HCAN’s list of congressional supporters. It includes Barack Obama (and Joe Biden). But it also includes … John Conyers (who introduced H. R. 676, the single-payer bill Vienna was writing about).
Single-payer advocates who sign on to Obama/HCAN-style mixed public/private plans will have to push for certain key elements to make it into the final legislation, like (for example) a public option (“Medicare for anyone who wants it”) and strong regulation on private insurers prohibiting discrimination based on pre-existing conditions.
For better or for worse, the mixed plans have a lot of momentum right now. The biggest domestic health care event of the week was Senate Finance Chair Max Baucus unveiling his new white paper. He isn’t an HCAN supporter, but  his plan is in the Obama/HCAN vein. Very roughly, it’s Obama plus an individual mandate.
For more healthcare activism, check out Families USA’s Stand Up for Healthcare, including their excellent blog, and long-time single-payer advocates Physicians for a National Health Program, which Lynn Moses Yellott was right to highlight.