Seems like there are at least one js engine based on canvas coming out every day.

I have been working on a pirate game based on canvas at http://constantsail.com , and I have to say, it is so simple to work with canvas that there doesn't seem to be a huge reason for these game engines. If you need canvas to do something that it doesn't you just tack on a function.

I guess the thing is, it is so easy to start building a game engine, everyone is starting to do it. I find it is easier to just start with the basics, and when you start to repeat yourself or do anything complicated, just encapsulate it and keep building.

I built a knockoff of atari combat the same way on the iphone (http://robkohr.com/iphone/tank)