Sketch for a 'watch' function on d3 selctions...

circles.js

var data = [ ],
    width = 500,
    height = 500,
    id = 0;

function addRandom() {
  data.push({
    x: width * Math.random(),
    y: height * Math.random(),
    r: 10 * Math.random(),
    id: id++
  });
}

setInterval(function() {
  if (data.length == 10) {
    data.shift();
  }
  addRandom();
}, 100);

var svg = d3.select('body').append('svg')
              .attr('width', width)
              .attr('height', height);

var key = function(d) { return d.id; };
var circles = svg.selectAll('circle').watch(data, key);

circles.exit().remove();
circles.enter().append('circle');
circles.attr('cx', function(d) { return d.x; });
circles.attr('cy', function(d) { return d.y; });
circles.attr('r', function(d) { return d.r; });
circles.style('fill', function(d) { return 'red'; });

watch.js

d3.selection.prototype.watch = function(data, key) {
  var selection = this,
      watch = {},
      exit = {},
      enter = {},
      watchCommands = [],
      enterCommands = [],
      exitCommands = [];

  watch.attr = function(name, value) {
    watchCommands.push([ 'attr', [ name, value ]]);
    return this;
  };

  watch.style = function(name, value) {
    watchCommands.push([ 'style', [ name, value ]]);
    return this;
  };

  watch.enter = function() {
    return enter;
  };

  watch.exit = function() {
    return exit;
  };

  exit.remove = function() {
    exitCommands.push(['remove']);
    return this;
  };

  enter.append = function(selector) {
    enterCommands.push(['append', [ selector ]]);
    return this;
  };

  d3.timer(function(){
    selection = selection.data(data, key);
    var exitElements = selection.exit();
    if (!exitElements.empty()) {
      exitCommands.forEach(function(cmd){
        exitElements[cmd[0]].apply(exitElements, cmd[1]);
      });
    }
    var enterElements = selection.enter();
    if (!enterElements.empty()) {
      enterCommands.forEach(function(cmd){
        enterElements[cmd[0]].apply(enterElements, cmd[1]);
      });
    }
    if (!selection.empty()) {
      watchCommands.forEach(function(cmd){
        selection[cmd[0]].apply(selection, cmd[1]);
      });
    }
  });

  return watch;
};