The new custom fields hook is very cool. The continue the idea of making custom fields more flexible (more CCK-like?), I would suggest an option to define the Multiple-Choice options for a custom field by PHP snippet, rather than requiring a static list.
This would allow, for example, the possibility of creating a 'Node association' type field, which would be populated with all Drupal nodes of a certain type. Now I can associate a Contact directly with their "Resume" node, for example, on my theoretical business networking site.
This would be especially valuable in concert with an autocomplete input type, when there is the possibility of returning a large number of options.