Sonnet has only been offering DIY CPU-upgrade kits for mac users up until now. If you've got an old 550mhz or 667mhz TiBook around and $500 to blow, you're set. Sonnet will now offer a 1.2ghz upgrade for the TiBook with 512k of L2 onboard cache. Sonnet insists they do the upgrade though do the complexity of the powerbook. Trust them on this.
My powerbook is in shreds from replacing the hard drive myself. They'll also test it to make sure it's working 100% and will ship it back to you right away.