Putting this in the CSS makes checkboxes vertically aligned in Firefox.

input[type=checkbox] {
  vertical-align: middle;
  position: relative;
  /* bottom: 1px; */
}

I haven't tested other browsers yet, but presumably only IE6 might be a problem. And even then, it would only be a minor layout issue, so I'll leave it at that.

I got the answer here, where there is more detailed info about cross-browser solutions:
http://stackoverflow.com/questions/306252/how-to-align-checkboxes-and-their-labels-consistently-cross-browsers