By Aidan Malley

Published: 07:35 PM EST

A clause in the license for the latest server edition of Mac OS X will let the software run outside of a fully native environment -- and developers are relishing the prospects of supporting virtual Macs for the first time.

The small but significant extension contained within the new software agreement (PDF) is the first sign that Apple is no longer insistent on a one-to-one ratio for Mac OS licenses to Macs, as it was with Tiger and all previous versions of the operating system. In the updated usage terms, a theoretically unlimited number of licenses can be in use as long as they remain valid.

"You may also Install and use other copies of Mac OS X Server Software on the same Apple-labeled computer," the new clause reads, "provided that you acquire an individual and valid license from Apple for each of these other copies of Mac OS X Server Software."

The agreement does not extend to the regular Leopard client and still requires a full paid license for each virtual machine, a move meant to discourage law-abiding users from running a multi-license copy of Mac OS X on a single computer.

Even with this restriction, the modification is already considered a watershed moment for Apple's efforts in business by its professional customers and those writing virtual machines, including Parallels developer SWSoft. The latter's Director of Corporate Communications, Ben Rudolph, notes that the ability to run one or more extra copies of Mac OS X on an Xserve computer could be a decisive factor for switching some Linux- and Windows-based server environments to the Mac.

Many of these businesses need to separate programs from the main operating system in the event of a malware infection or a crash, or else need a sandbox to test new software without buying an entirely separate computer. This is already commonplace with Virtual PC, VMware, and similar tools on most operating systems, but until now has been impossible with Apple hardware. This will change in the next several months when SWSoft intends to release Parallels with its first instance of Mac OS X virtualization support, Rudolph says.

"It is important to note that we’ve already begun the steps necessary to technically enable this new policy and Leopard Server is an important part of our Parallels Server roadmap," he explains. "We know from many of you that the 'holy grail' of Xserves is to run multiple, isolated, near-native instances of OS X Server on the same box, at the same time. Couple that with the ability to run Windows and Linux next to those instances of OS X via Parallels Server, and you’ve just made Xserves even more compelling for enterprises large and small, even non traditional Apple shops."

This has prompted speculation that Apple may be pressured to upgrade its Xserve rackmount system, which (along with the Mac Pro) has largely been left untouched since its debut in August of last year. Each virtual machine consumes a large amount of bandwidth, memory, and processor power, with more cores and memory often directly linked to more simultaneous copies. An eight-core or greater system could be essential to gaining a foothold in a business market that relies more and more on virtual operating systems, according to technology analyst and columnist John Welch.

"Apple doesn't yet make a box that's big enough to be an effective VM server for more than a handful of VMs if they're heavily loaded," Welch warns.