ARM machines aren't nearly as common as x86. Porting to ARM would be silly.
Depends on what you want to target.
In terms of netbooks, ARM is going to be big next year with Smartbooks running around Tegra and Snapdragon based platforms (ARM11 and ARM Cortex A8/A9). It seems like a very sensible platform to target to me. That's before all the smartphones, but I think that Android, WebOS, iPhoneOS, Maemo, S60 and even WinMob have this market tied up for now.
Imagine a $300-$400, dual-core, 1GHz+ ultra-portable laptop, running AmigaOS 4.5 reliably.
