From Joomla! Documentation
How do you set a predefined value?
I'm new with Joomla 1.6+ (but not programming), so I guess is very important to mention JFormField::setValue as the way to predefine de "selected" input value as a heritage of JRegistry. At least is the way I'm doing it :-S
[UPDATE]: I can see in libraries.joomla.registry that setValue is deprecated :-( but anyway the new function "set" is not mentioned