I have been skimming the debian-powerpc mailing list, trying to help with my limited knowledge, but of course, we could use some help.
Right now, it seems that one of the biggest problems regarding the distribution is to get some machines (e.g., the New World powermacs) with a proper bootloader, as the version of yaboot that is in Debian was last updated in 2006.
Some people have been trying the GRUB2 bootloader on these machines and it works to some extent (but it is still not close to having everything automated and ready to become the default bootloader on New World powermacs).  I don't have familiarity with IBM powerpc boxes, though.
That doesn't cover the other powerpc systems that need to use some lesser-known bootloaders like miboot, quik, BootX, or uBoot (the first 3 for Old World macs and the latter for embedded powerpc devices---of which I have a lovely KuroboxHD NAS, kindly donated by a fellow Debian Developer).
Another problem is getting X to work without problems.  I hope to do some extra work during this year's DebConf 10. That being said, Linux runs very well even on weak hardware, by today's standards. 
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