Likening the current Linux kernel to V4-V7 Bell Labs. UNIX on a PDP/11 is like saying that your Ford Mondeo is a re-implemented Model T.
It's true, it has four wheels, a motor, and used a steering wheel, brakes and a gearbox, but there ain't no compatible parts!
Now I'll defend V6 or V7 as being brilliant for their time until the cows come home, but don't suggest that Linux is 'just a re-implementation'. Even if you did, SVR3.2 with TLI or BSD4.4 would be a better reference than anything that ran on a PDP/11 (think communication subsystems).
And anyway, why should Android not be multi-tasking (OK, I'm blurring the distinction between timesharing and multi-tasking), but both the timeslicing and the privilege protection is just as useful in a mobile device as it is on a multi-user computer. After all, the inability of the iPhone and iPad to multi-task is one of it's biggest criticisms, and having an ineffective security model was seen as Windows biggest problem. Multiple tabs in Chrome on ChromeOS will probably be implemented as threads which will probably need to run in parallel, anyway.
Whilst BeOS, OS9 and VxWare people may think their OS's have significant advantages over a Linux kernel, Linux is not such a bad place to start. The API's are well understood, the code is open, and you can comfortably remove the overhead you don't need (I remind people that vanilla V6 UNIX on a PDP/11 without separate I&D space HAD to fit in less that 56K of memory!).