First, note that "ARM" encompasses a HUGE number of different chips, ranging from 28-pin $2 chips about equal to the mega168 (8k/2k/20MHz) (only w 32bit registers/operations) to chips running about 1GHz with external ram/memory (and capable of running linux.) Part of the confusion is figuring out the right development environment for whichever sweet spot you are interested in.
Some of the more "interesting" ARM development boards I've seen:
"Beagle Board" http://Http://www.beagleboard.org
($149!) A high-end (Cortex A8) linux capable board. Open Source (!!)
Luminary Micro Cortex-M3 Eval Boards: http://www.luminarymicro.com/products/evaluation_kits.html/
Luminary was one of the first vendors to address the potential of ARM in low-end microcontroller apps.
ST Electronics STM32 "Primer" boards http://www.stm32circle.com/hom/index.php
Cute (and pretty cheap) hand-held units featuring their STM32 Cortex M3 microcontrollers. There's an associated contest, too.
There are a bunch of boards from sparkfun/Olimex/etc for the NXP LPC21xx ARM7 microcontrollers. Not generally as "aimed" as the above.
There are numerous consumer appliances that include a core ARM/Linux system that can be modded to be re-purposed ... Most famous are the wireless routers (see DDWRT, OPENWRT, and the Fonera info here on the Arduino Forums.) But also things like the Matel "Juicebox" media player (now hard to find, but...)