Conditional profile elements.

It would be nice to be able to specify profile elements that only show up based on answers to other profile elements (ideally AJAX or AHAH style). Some examples:

1) An event registration might ask if translation is needed for a participant who has specified that his native language is other than the event's language.
2) An event that has an option to include lunch might only ask for menu choices if a participant has chosen that option.
3) One might ask for a spouse's name if one specifies marital status as married.
4) One might offer additional registration options based on membership status (a patron might be offered an option to attend the Trustees' Dinner).