Multi-tasking was made available in iOS last November, but I was surprised to learn that there is a hidden feature known as multi-tasking gestures that allows you to quickly switch between applications. Though Apple hasn't officially made multi-tasking gestures available for the masses, there is a way to turn on this iPad feature and it doesn't involve jail-breaking. You'll need an Apple computer capable of running the latest version of Xcode and you'll need to purchase and download Xcode, which will cost you $4.99. The download and installation of Xcode will take 10 or 15 minutes, but once you have all these pieces enabling multi-tasking gestures takes about 10 seconds.
Connect your iPad to Xcode and click on Use for development. Xcode will ask for your iOS developer credentials, but just click on cancel. You'll see an error message which you can ignore. Disconnect your iPad and you'll see a new option in Settings->General.
The new gestures made a big difference for me when using the iPad in the cockpit. I use Penultimate to write down clearances or record the surface weather, but then I switch to an approach chart viewed in SkyCharts Pro or some other chart viewer. Problem is, I often forget something that I've written down and before multi-tasking gestures I found myself fumbling to get back to the notes I'd written. Now I simply swipe four fingers horizontally and voila! There are my notes. Rather than having to swipe left or right, I do wish that the scrolling between the apps was implemented with a circularly linked list. That way one could just keep scrolling left or right and cycle continuously through the apps.
Apple is bound to enable this feature for everyone eventually, so you can always wait for the next release of iOS if you don't want to go through this process. Overall, I find this hack dramatically improves the iPad's usability in the cockpit. Check it out!