One of the big mysteries about Windows 7 has been whether Microsoft would offer a "family pack" license letting home users install the operating system on multiple computers for less than the cost of buying copies individually. Then, last week, tech enthusiast Kristan Kenny uncovered language in a licensing agreement signaling plans for such an offer.

Still there were skeptics. Blogger Adrian Kingsley-Hughes initially sought to debunk the rumor. But a day later, his fellow ZDNet scribe Ed Bott uncovered even more evidence of Microsoft's plans -- in the form of legitimate online retailers prematurely listing a Windows Home Premium Family Pack Upgrade with a great deal of plausible detail. Kingsley-Hughes, who initially said his contacts in the know hadn't heard about such an offer, has now concluded that the rumor "does indeed seem to be genuine."

Even Microsoft's official "no comment" hints at something in the works.

“We will continue to work with our partners and expect to have other great offers in the future as we lead up to and beyond general availability," a spokesperson said in reponse to our inquiry about the Family Pack. "We have nothing to announce at this time."

So what does this mean? First, it's important to note that the prices uncovered so far are for upgrade versions, which let existing Windows users move to the new Windows 7 for less than if they were starting from scratch. Based on the online listing, it looks like a Windows Home Premium Family Pack Upgrade would be priced around $150, for installation on up to three computers.

That compares to a $119.99 price for a standalone upgrade to Windows 7 Home Premium. As noted by Gregg Keizer of Computer World, that means Family Pack upgrade price would save someone about $210 compared with buying the upgrades individually.

In a presale promotion scheduled to end Saturday, Microsoft and selected retailers are offering individual Windows 7 Home Premium Upgrades for the discounted price of $49.99. But after that point, the standard $119.99 price is scheduled to kick in, and a Family Pack would start to look very economical for people who need to install Windows 7 on multiple computers.

By comparison, Apple says it will offer the family pack of its upcoming Snow Leopard Mac OS X for $49, as an upgrade from the existing Leopard operating system, for installation on up to five computers. Upgrading a single Mac from Leopard to Snow Leopard will cost $29. Snow Leopard is due in September.

The down side of a leak, as opposed to an official announcement, is that we don't know exactly how Microsoft would offer the Family Pack -- who would be eligible, and under what terms and conditions. Until those details become certain, it's tough to say exactly how meaningful the offer will be, and it's just as tough to factor it into buying decisions.

But whatever form it takes, at this point it's a pretty safe bet that a Windows 7 Family Pack is coming.

Windows 7, an all varieties, is slated for release Oct. 22.