Nope, FB is a separate install which has been available to developers since before 10.8 was released.
It's available to developers via a separate install at the moment because developers need it to test integration with their apps, but Apple will bundle it with a point upgrade. Almost certainly 10.8.2. By bundling it with a point upgrade, Apple only need worry about one release, for support and distribution. All point upgrades are made using Apple's app installer mechanism under the hood anyway. Apple are currently distributing 10.8.1 using this mechanism.
Any chance 10.8.2 adds support for running iOS apps?
Not a chance. Apple couldn't even if they wanted to. iOS apps are built specifically for ARM processors, so would run unbearable slowly on desktop processors. Also, Apple's been careful to separate apps built for touch devices and desktop. They're not going to break that.