Well, the first generation of android cellphones ran on ARM11. My previous phone, the HTC Hero, ran on a 500mhz ARM11, and it mostly ran a seriously underoptimised OS (android 1.6). The first wildfire also ran on the same chipset.

Comparing the OSes, maemo 5 (and I presume harmattan) is way, way more optimised than the early android versions. By getting rid of the android java VM, clocking the CPU at maybe 600mhz, adding of course half a gig of RAM and maybe making the UX lighter on effects and such, I have a gut feeling that the experience would be more than passable. That Hero could do some seriously impressive stuff, considering its hardware. If it ran Maemo 5 or Harmattan and was priced at 250 euros, I'd buy it today in a heartbeat.