I hate to be the person to bring Windows 8 up; but I should, because as you do, I agree it is an OS issue. Windows 8 is arguably the only existing OS designed from scratch for tablets, while others were adapted from smaller form factors. Coming from around 3.5" form factor, design concepts are obligated to be full screen. and when OS practically stayed on the same concept, even with modern tactile apps, developers are left with no options.
Unlike you, I don't think tabbed interface is the proper answer for the tablet realm, because what keep me on my tablet is mostly when I need to do two totally different things: ssh to a server and have a web page open next to me. Chat with someone, while reading a pdf document. I think one of the best examples of multitasking is implemented in Windows 8; although I don't use it.
Another type of effective multitasking is Facebook chat on android.
In the mean time, I have to stick to my laptop, for a bit longer.