Funny you should ask, this was actually a question that Robert Morris asked on my YC application. I don't view Heroku as a competitor. In fact, I believe they announced that they abandoned their node.js experimental hosting.
However, Joyent really has a leg up on node.js hosting, and seeing as they employ Ryan Dahl and just basically bought node.js, it could be difficult to gain market share, but I believe there is room for others to play.
Actually, Heroku has 1,000 people running node.js apps on their platform. The first phase of the experiment is over, but they're releasing the new version of it next year apparently.
I stand corrected, but still feel Joyent is the major player at the table with four aces as of now. Plus, it will be interesting to see what salesforce.com does with the direction of Heroku.
However, Joyent really has a leg up on node.js hosting, and seeing as they employ Ryan Dahl and just basically bought node.js, it could be difficult to gain market share, but I believe there is room for others to play.