All depends what you want.

I just took a bit of time to read some CAN stuff (I've never worked with it) and I must say it sounds like a great technology.

From what I read, though, you couldn't do it via RS485 though, because it requires a recessive/dominant bit at the physical layer - ie something like an open collector output as used by I2C bus for non-
destructive arbitration. Maybe someone with more experience can clarify.

What kind of cost is a bog-standard CAN driver and how does it hook up to a '51? A full 8 bit port? Or would a variant with the built in i/face would be better? What's the cost of that?

Drawback of 485 is that, although you can in theory put lots of devices on a bus, you need some way of arbitration. It's not built in to the standard. Whether that is a problem or not depends on what kind of topology you need. (A ring of point to point connections for instance is very do-able with 485.)

How is the actual physical interface done for CAN? Is it actually a pair of o/c outputs, 120 ohm terminated, one pulled up, one upside-down? (ie pulled down)?

What if you are in a harsh environment and you want isolation?

I must say if I had a project that would suit CAN right now I'd think strongly about it. Looks great.