There was a recent panel discussion at UC about MySQL, scaling up or out, I think there was a link on YC to a noteset, with some pretty big names. There was a round of questions about server specs. WRT OS, they all ran OpenSolaris or Linux (specifically RHEL, Fedora, Ubuntu, or SuSE)

I think this is the link that came over the wire http://www.paragon-cs.com/wordpress/2008/04/16/scaling-mysql...

but this seems to have the best aggregation of notesets from the panel, from tables to video and lists: http://www.technocation.org/content/panel-video%3A-scaling-m...