Regardless of the fact that you're in the MacBook/MBP forum, I'll try to help:
The aluminum on PowerBooks (like on the MBPs) is anodized, thus making "polishing" (especially when concerned with more than cleaning crap off it) exceptionally difficult if not impossible.
If it's scratched and dull, you're likely not going to get much results with anything less than replacing the affected parts, but perhaps someone has a better solution.
I don't recommend polishing the aluminum enclosure. To polish off the imperfection on the surface means grinding out the surrounding areas of that imperfection. If you polish out the scratches off our PB, you are scrapping off the perfect areas of PB's casing to make it even with the scratched part.
The better way would be the preventative measure so that you don't introduce further scratches by getting the micro-fiber sleeves from Radtech or Marware.