I personally find the skills required to develop old C64 games quite useful, as it allows people to understand not only the high-language useage but also low-level programming as well in order to squeeze more performance out of what they have.
Some very good skills that while might not be widely used today allow people to learn the skills that help them to programmer more wisely and understand the fundimentals of what is going on behind the scenes.
But then again that's just my view, and I'm sure there's a huge number of people waiting in the wings to disagree.
I don't know of any tutorials for the system, but I'm sure that some of our older folk around here who used to love the system wouldn't mind working on some code if you asked them. I'd provide some, but it'd mean learning C64 BASIC I never programmed on the machine; only used a friends from time to time.
Intel Core 2 Duo E6400, 512MB DDR2 667MHz, ATi Radeon X1900 XT 256MB PCI-E, Windows Vista Business / XP Professional SP2