Nothing new here. N900 and N9 can do it since always. Normally Android suspends the running app when another app (activity) is activated, just like iOS and WP. However, you can create background services on Android and do whatever you want, which is something iOS and WP won't allow. (and why shouldn't buy those, amongst many, many other reasons).

On the N9/00 apps will run in the background, unless the developer decides otherwise. Just like a normal PC (However, Windows 8 is also idiotic in that regard). On the N9 at least you can even distinguish between active, visible in the taskswitcher and running in background, so you can actually see multiple apps doing stuff at the same time in the task switcher.

@Chuck Norris: it is about background tasks. The ability to stream music to the radio, running Drive in the background with spoken navigation while your spouse is browsing the web on one and same device.