For those with pencil and pad handy, that’s a four-month delay announced two months before a major operating system update was supposed to ship. But for investors, the important thing is that this could cost Apple hundreds of millions or dollars in revenue this fiscal year.

Apple apologists will point out that operating system delays happen all the time. Microsoft (MSFT) is known to deliver operating systems months (OK, years) late. The modern Apple has been much more reliable. It was CEO Steve Jobs, after all, who managed to finally get OS X out the door, complete with Aqua interface, after years of empty promises. (Let’s not get into Copland.) So let’s give Apple credit where credit is due. Jobs & Co. usually don’t subject us to this sort of thing.

Still, there will be consequences here, beyond the embarrassment of tardiness. Apple has long positioned Leopard as a superior alternative to Windows Vista, but for the laptop-heavy Dads & Grads season, which Apple would have caught with a June release, Leopard won’t be an option at all. That means computer buyers will have to decide whether to (1) buy a machine with a Mac OS they know will be obsolete in six months, (2) wait until October, or (3) buy a Windows PC. (Mac lovers won’t buy a PC, but Windows folks who were on the fence about a switch to the Mac? They just might.)

This delay will carry financial consequences – both for Apple’s fiscal fourth quarter, which typically ends in late September, and for its fiscal year. If Apple had been able to ship Leopard on time, it would have benefited from hardware sales and high-margin software sales for the entire three-month period. Now it will get none of that benefit for the quarter – and therefore none for the fiscal year, which will end just before Leopard ships.

How much money is Apple leaving on the table? When Apple released OS X Tiger in April 2005, it got a 44 percent sequential bump in software sales, which was a 64 percent year-over-year increase. Apple typically pulls in about $300 million each quarter from the sale of “software, services and other sales,” judging by a look at the company’s financials. So it’s conceivable that this delay could cost Apple $100 million of high-margin sales in the fourth fiscal quarter factoring in software alone. There clearly will be costs from people who choose to delay their Mac purchase until Leopard comes out; I’m personally waiting to buy a new Mac for myself, and one for my mother. I would have gotten them in June, but now it looks like October. I’m guessing that in total, Apple might miss out on $250 million in revenue it might have otherwise booked during a fourth quarter that’s likely to see sales above $5 billion total; that’s not a trivial amount.

(It’s most important, however, that Apple get Leopard out during October as planned – and as early as possible. The first fiscal quarter is Apple’s biggest, what with the holiday season and all; the company will want to have a fully refreshed line of hardware and software to tempt shoppers for the entire stretch of time.)