As speculated, during its earnings call today Apple revealed that Mountain Lion would be released 25 July, US time, following the exact same pattern as its Lion release.
CNET's Josh Lowensohn reported:
As a quick recap, the US$19.99 software update brings a handful of iOS features to Macs, including the notes and reminders apps. It adds a few other things, like Twitter integration, Apple's Game Center and iMessage services. There's also a new security feature called Gatekeeper, designed to fend off malware by controlling what applications can and cannot be installed.
Mountain Lion will be available as a download only at the Mac App Store.