What are the limitations of Task Finishing in iOS 4?

Task Finishing is one of the multitasking features of Apple’s iOS 4 for iPhones, iPods touch, and (eventually) the iPad.

Apple describes Task Finishing as:
“If your app is in mid-task when your customer leaves it, the app can now keep running to finish the task.”

Anandtech provides slightly more detail:
“Task finishing is the most traditional multitasking service that iOS 4 offers. You can mark a process using task finishing and that task will continue to run even after you exit an app for a finite period of time. This would allow for a chat app to continue to receive, update and log messages even when you’re not looking at what’s being said.”

Is the “finite amount of time” documented somewhere? Is it a fixed amount?

iOS devices typically have a limited amount of system RAM available to apps. When that RAM is needed, the “background” apps get bumped off. A good example of this is “Fast App Switching” where you can switch quickly between a couple of apps… however, if you’re doing something RAM intensive, like opening a bunch of heavy sites in separate pages in Mobile Safari, then any backgrounded apps might get shut down completely such that they must relaunch (that is, the way things worked in earlier versions of the OS).

With that scenario in mind, is Task Finishing limited in the same way as Fast App Switching? That is, is there a guarantee that the task you flag will actually finish? Or might it get bumped from the background if system resources are otherwise required by the foreground app?