iPhone OS 4 beta 2, released yesterday, has introduced a new, circling, side-switching animation for multitasking fast app switching. The new animation occurs when you switch between two apps either via the new, double-click-Home to trigger to launch the multitasking UI, or when one app calls another app (i.e. when you're in Contacts and you tap to send a contact an SMS, as noted by @ranger_bill).
Launching or leaving an app retains the same, zoom-based effect as always (though the wallpaper in iPhone OS 4 zooms slightly as well, like on the iPad -- gotta love those little details Apple puts in).
Check out our complete iPhone OS 4 preview and feature walkthrough for more and hit up the video after the break!