I was going to write a novel size post, but what the heck.
I see that people have already tested bitcoin mining on different ARM platforms including the Nokia N900 with omap3. My question is, were these tests done with platform specific optimizations? ARM based handhelds are insanely powerful esp. from the efficiency point of view. Relatively cheap devboards for TI OMAP platform are available [1][2].
GPU and FPGA hardware fail to do scrypt because of their slow memory access. Now, the TI omap SoCs feature advanced SIMD engine called NEON with direct access to L1/L2 cache. According to wikipedia, NEON does MP3 decoding at the frequency of 10MHz. These chips can go up to 1.2GHz!
I am not an expert so I had better quit talking. Anyhow, I think OMAP's have much potential to unleash.
Links:
http://e2e.ti.com/support/dsp/omap_applications_processors/f/447/t/30265.aspx
http://wiki.debian.org/ArmHardFloatPort/VfpComparison
http://en.wikipedia.org/wiki/ARM_architecture#VFP
And BTW, I have an OMAP3 board (from "the worst product ever made" called Touchbook, basically old beagleboard hardware). It's dead now, won't boot from SD anymore, serial port might work. I'm donating it to the first person who knows how to get it to boot from USB.