> there's no other commonly available PPC computers that make sense porting to (yet).

G5 Macs anyone? :-)

> Does anyone know where Fixstars' motherboards came from?

The PowerStation came with IBM's "Bimini" board. For "Bimini" refer to:

http://blemings.org/hugh/blog/blosxom.cgi/2007/08/05
http://blemings.org/hugh/blog/blosxom.cgi/2007/09/29
http://blemings.org/hugh/blog/blosxom.cgi/2008/06/10
http://www.penguinppc.org/news/2007/08/10/970MP_workstation
http://www.power.org/news/pr/view?item_key=e6edc76b12015b289dbc18089d2d7cd556a4f83d
http://web.archive.org/web/20071017044111/people.freedesktop.org/~idr/blog/index.php
http://www.phoronix.com/scan.php?page=news_item&px=NjA3MQ
http://lisas.de/~adrian/?p=46
http://lisas.de/~adrian/?p=45
http://lisas.de/~adrian/?p=44
http://lisas.de/~adrian/?p=6
http://www.ibm.com/developerworks/power/pa-slof/
http://www.ibm.com/developerworks/power/pa-slof-js20/

Later, Genesi's Matt "Neko" Sealey revealed that the "Bimini" is IBM's further development of Genesi's/bplan's TetraPower board, which was supposed to come with their Open Server Workstation (OSW):

http://www.powerdeveloper.org/forums/viewtopic.php?p=12748#12748

For TetraPower and OSW refer to:

http://www.powerdeveloper.org/platforms/tetrapower/specifications
http://www.powerdeveloper.org/program/tetrapower
http://www.powerdeveloper.org/downloads/OSW_Block.pdf
http://www.flickr.com/photos/54643129@N00/112833476/
http://web.archive.org/web/20071014173313/http://www.genesippc.com/files/power.org/DP-WG.pdf

Historical "review" of TetraPower/OSW:

https://morph.zone/modules/news/article.php?storyid=925
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4095&forum=11&start=20#38077
http://www.morphos-news.de/index.php?lg=en&nid=1201&si=1
http://www.amiga-news.de/en/news/AN-2006-02-00086-EN.html
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4498&forum=11&start=40#42280
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4508&forum=11
http://www.powerdeveloper.org/forums/viewtopic.php?p=2893#2893
http://www.amiga-news.de/en/news/AN-2006-07-00046-EN.html (TetraPower as reference design for PAPR)
http://www.powerdeveloper.org/forums/viewtopic.php?t=754
http://www.powerdeveloper.org/forums/viewtopic.php?t=819
http://bbrv.blogspot.com/2006/11/watts-happening.html (bottom)
http://www.genesi-usa.com/press/2006/11/6/