so su me
 

how does KDE do that thing where when they need root permissions they ask for the password and then run the exec? how can one do that programmatically (yes, I could just look at the source, but there's just so much of it!)?

There is a program called kdesu that does what you are talking
about. If you look in the properties for such a program
dispatched from the desktop or menu, It probably has something
like 'kdesu <program>' in the execute field. So you are actually
running the kdesu program with the <program> as an argument.

Please take a look for yourself, what may be executed may be
different, but I think that the idea is the same.