Very interesting to play with but also very dumbed down (see ios). Will have its greatest marketshare gains in the under 20 crowd and may grow with them.
webos, blackberry, etc (in their current forms)
It would take a major shift from one of these to overtake android. With google on the forefront of the software side and the combined R&D of Samsung, Motorola, LG, etc on the hardware side - it would be hard for one of these to pop up.
The winner will have to be open source/cross hardware, etc.