I did some experimentation over the weekend. We haven't officially worked on ARM yet but it didn't take much to get this going. This is SmarterMail running on a $23 ARM board and a usb drive.

I had this running on battery for a little while at the office showing it off. A SmarterMail running off wifi, battery, and a USB drive. The evolution and changes we've made to SmarterMail has opened us up to a whole new world of hosting, its pretty cool!