From what I understand WebOS and Android don't allow for the development of software as robust as Midori.. even though they are Linux based, it cannot be easily ported. Opera Mini is in the works for Android though, which, as far as I know, is the first alternative browser for the OS. I know next to nothing about WebOS though. Android for instance supports applications written in Java only.
Midori does work on Mer
(was formally default browser, now Tear is) which is a mobile OS that is compatible with some cell phones (OpenMoko Freerunner) and other mobile devices like the Nokia Internet Tablets and the Q7. Midori is also available for Maemo, which runs on the Nokia N770,N800,N810,and N900.