Conditional defualts

I don't think this is already implemented (or I may be just a moron ;-) )

I'd like a default value depending on controlling field.
For example I may have a "field_user_is_internal" controlling field and then a "field_organisation" depending field that I would like to set default to "My organisation" (and hide) if the flag is true.
But I suppose it would be big job... ;-)