Check out pop up multiple window multitasking from Paranoid Android23
The video shows the Paranoid Android multiple window effect on both a Nexus handset and an Android tablet. Notice how the notification shade is used in this process. The developers explain that the video shows less than 10% of the functionality of the program. If so, looks like this is worth keeping an eye on for the future. One thing is for sure. These guys don't lighten up when talking about the competition.
source: ParanoidAndroid via Engadget