In a very non-Apple move, Jobs revealed today that they would be releasing rack-mounted servers next week.
This eWeek article provides more of the limited details available on these upcoming machines:
Excluding a glimpse at a brushed-metal faceplate resembling the Titanium PowerBook, Jobs declined to provide additional details about the "major release" of a dedicated, rack-mounted server until its unveiling May 14. Nevertheless, he did specify a few features of the new server OS that will accompany it. Besides LDAP, NetBoot and NetInstall support, the server will be able to run headless. It will also feature a server-optimized Java Virtual Machine; disk, print and mail quotas; and support for Python, Fast CGI, YCC and Ruby.
The oldest MacRack rumor available is from November, 2001... which also provided the most details: claiming a 1U rack-mounted Mac starting at $899.
More recent rumors, however, lack any specifics.
Close attention will be payed to the upcoming rack-mounts as they would certainly hint at technologies soon to be available in the Pro machines. The recent emergance of a Quicksilver server motherboard on eBay might hint at the future specs...