As Tom explains, although Photoshop CS5 & Lightroom 3, following Apple’s lead with Snow Leopard, no longer support PPC, the team has maintained support in the DNG Converter. That way customers who couldn’t upgrade to CS5/LR3 could convert their files to be backward compatible.
Supporting aging systems comes at the expense of enhancing the apps for the future, and the overlap between people using the very latest, high-res SLRs together with 5+ year-old Macs has gotten small. Thus 6.4 will be the last PPC-compatible release of the converter.
Just to be clear, DNG files themselves are unaffected, so in the future you could use an Intel-based Mac (or a PC) to convert proprietary formats for use with PPC-based Macs.