This page lists the state of ppc64 support in common Linux Distributions
SuSE Linux Enterprise Server (SLES) version 9 has out-of-the-box support for ppc64 processors
Redhat Enterprise Linux (RHEL) version 3 supports ppc64 processors
Yellow Dog Linux
Yellow Dog Linux is planning a 64-bit ppc distribution which will be released in May 2004; a beta download is available from ydl.net
A ppc64 version of Debian is under development at the moment
This distro is entirely 64 bit from top to bottom.