Thank you, Kevin.
I DID understand the question, and also see value in the iPad way of doing it. You can quickly switch between two apps -- back and forth, back and forth -- easier than most Android paradigms. The ICS recent apps feature, with its button, is a nice step in a similar direction.
I just realized that maybe an alternate launcher couldn't do this with a gesture, because the launcher apps are activated by "Home," right? You'd need something that intercepts gestures on any screen, in every app. HTC Sense echoes the "recent apps" on its Notification window shade, but ... that's still not quite what OP is asking for. Frankly, with the way Apple is bullying big competitors, I don't see a humble little under-funded developer taking a chance on duplicating an iPad convention and losing his house, car, retirement, and livelihood in a legal defense, or settlement, with Apple.
I test software for a living, so little things like this a) get my attention and b) are interesting for me to analyze ... to a point.