Update: During its quarterly earnings conference call this afternoon, Apple has confirmed that OS X 10.8 Mountain Lion will go on sale tomorrow Wednesday, July 25. It will be available early morning through the Mac App Store for $19.99.

Apple has announced that OS X 10.8 upgrades will be free for qualifying Macs purchased at authorized resellers on or after June 11. The offer will be good for only a short period of time though. The deal ends just 30 days after Mountain Lion is made available in the Mac App Store, so users are encouraged to snatch the upgrade as soon as they can. Mac owners will be able to redeem the deal here.

What defines a "qualifying" Mac is not made clear, but it seems reasonable to assume any new or Apple-refurbished Macbook, iMac or Mac Mini purchased after June 11 from Apple's website, one of its retail stores or an authorized reseller is probably as stringent as the fine print gets. Resellers include the likes of big box stores like Best Buy and Macmall but can also include smaller, local shops too. Amazon, J&R and MacConnection are on the list as well. 

The "Up-to-Date" program, as Apple refers to it, is analogous to past Microsoft offerings where free Windows vouchers have been given to new PC customers just prior to a new release. Unfortunately for PC users, it looks like Microsoft will be charging for Windows 8 upgrades albeit at a greatly reduced price of $14.99. Some PC manufacturers are promising to pick up the tab though, offering Windows 8 upgrades to their customers for free anyway.

If you don't plan on buying a brand-new Mac any time soon though, don't sweat it. Apple has also promised its customers an upgrade path to Mountain Lion for a mere $19.99. That's actually $10 cheaper than Lion upgrades offered last year.

Mountain Lion is expected to launch any day now while Windows 8 will be hitting store shelves October 26.