content validation re-sets node fields to defaults when field values not present

I was experiencing an issue with CCK Field Permissions where fields hidden from a user in a certain role were reset to defaults during validation when a user from that role edited a node. (specifically, on/off checkboxes) I ended up solving it by modifying content.module to load a copy of the original node, and cease modification of any fields that were not present in the form being submitted, but did exist in the original node.

I don't know how great my solution is, partially because I don't know how expensive a noad_load() is, and partially because while it's working for me, it's possible I broke something else. So, I'm just throwing this out there for people to review, and maybe integrate if it seems sensible.