Well, there's a developer preview of Mountain Lion available now, so that software makers can update their apps to take advantage of OS X's new features. As for end users like the rest of us, when it announced the developer preview via press release, Apple specifically said "late summer 2012." That's not a precise date, so Mountain Lion could leap into action any time before the Autumnal Equinox on September 22. For your historical reference, Lion arrived on July 20 last year, while Snow Leopard shipped on August 28, 2009--both those OS X updates were given initial release dates of "summer" as well.
How will I get my hands on Mountain Lion?
The same way you got a hold of Lion--via the Mac App Store. Apple plans to make the new version of OS X available only as a download from its online retail store.
Apple first started making its OS X update a Mac App Store-only affair with last year's Lion release. While the company hasn't released official details about Mountain Lion's rollout, it wouldn't be crazy to assume that many of the same details for that update's release will hold true this time around. Namely, Apple let you pay for and download one copy of Lion that you could install to multiple Macs--a welcome feature for multi-Mac households. Apple also invited upgraders to take advantage of the Wi-Fi in its retail stores to download and install the update--ideal if you've got a slow broadband connection or need help with the installation. Again, Apple hasn't confirmed that it's planning on reviving these aspects of the Lion upgrade process for Mountain Lion, but it certainly seems within the realm of possibility.
Apple later made Lion available on a USB stick. A report at Pocket-Lint.com indicates that the company doesn't intend to repeat this experiment, and Apple told Macworld the same thing. (Of course, prior to Lion's launch, Apple was insisting that the Mac App Store would be the only source for that update, too, so we'll have to wait and see if anything changes.)
Will I need to be running Lion to upgrade to Mountain Lion, or can I upgrade straight from Snow Leopard?
According to the information Apple has provided with the developer preview, you need to be running at least the latest version of Snow Leopard (OS X 10.6.8) to install Mountain Lion.
What version of Mac OS X is Mountain Lion?