COMPLETELY REVISED
Ubuntu for Android -- use tag: ubuntu-for-android
In every dual-core phone, there’s a PC trying to
get out.
Ubuntu for Android is designed to run on Dual-Core ARM-based smartphones. It is available from Canonical (Ubuntu 'owner') for OEMs to add to their smartphones (for sale). It is intended to provide the Ubuntu desktop experience, but based on the processing power, memory, and graphics capability of an advanced smartphone.
Currently, it has not yet addressed how to modify the interface, touch, memory usage, and other issues to better suit ARM-based mobile devices.
Ubuntu on ARM -- use tag: ubuntu-on-arm
wiki > Ubuntu Core on Nexus 7
Ubuntu for 'mobile' (tablets and smartphones, typically ARM-based) is currently under development and in early beta testing. Its current reference platform is the 'Google / ASUS Nexus 7' 7-inch tablet.
This product (for Nexus 7) is specifically compiled to run on the ARM processor, work with the current ROM / BIOS of this device, and to allow testing (and improvements) to "Ubuntu core" for this type of platform.
Any question about the future, other platforms, and cross-architecture support are probably too early. Also, any answers would be subject to change, technology, market share, and other factors ..
If you want a better answer, ask a single straight-forward question.