But a commercial version of L4 MicroKernel (http://www.ok-labs.com/products/okl4-microvisor) seems to be precisely targetted to smartphones and tablets

I.e a quadcore ARM cpu theorically can run four OS concurrently with performance like single OS on single core and without the need of specific virtualization hardware (that only Cortex A15 has).

Of course N900 is not a quadcore, but it is only a proof of concept and it should be nice to see a N900 which runs Meego/Nemo + Maemo + Nitroid + Ubuntu + ... concurrently (even if at slow speed).