A phone book app is a really good idea....but it would have to permanently replace the main ui one which for now is imposable. It would be nice to click the Phone button and the same bottom buttons remain (speed dial, contacts, history, and dialer.) but instead of going to the speed dial list it goes to folders which family friends business ect...I suggest you go to:
www.Instinct-software.com which is a application store for the samsung instinct and write a letter in the contact us section, I am sure they will be up to your ideas.
I do feel that the thumbnail feature which allows you to scroll by letter is really useful once you get the hang of it and start to know the location of the letters are you can get to a letter pretty quickly.
Just a side note... when I had a phone with a numeric dial pad (push buttons) I would separate my contacts in the same manner, by business, family ect. I would put F-in front of a family contact, a B- In front of a business contact and so on. This way if I wanted to quickly scroll to a certain category I could press 2 for B- Business contacts 3 for F- Family contacts and so on... I'm pretty sure you can do that with the Instinct and just scroll to that letter but that would be useless.....
