Just throwing it out there for discussion. Been reviewing domains in 0.7.x and really glad we are getting them! Good work.

One API thing at the moment has bugged a me a few times that I thought it would be worth a discussion regardless of the outcome (and at the very least would help me understand the current design choices).

Right now domain.intercept takes a callback that includes the err argument but from what I see in the code (https://github.com/joyent/node/blob/master/lib/domain.js#L142-177) that error argument will always be null if it makes it that far. Why not remove it entirely to give the user a tip off that intercept is actually taking care of that?

e.g.

var d = domain.create()
d.on('error', function (err) { /* handle errors */ })

fs.readFile('./foo', d.intercept(function (data) {
   // do something with successfully returned data
}));