MMC read errors

I'm seeing weird read errors on a DM3730. Has anyone seen anything like this before?

Environment:
factory images from
http://cumulus.gumstix.org/images/angstrom/factory/2011-08-30-1058/
loaded onto and MMC card on a
Gumstix WaterStorm on a Chestnut board

Commands:
dd if=/dev/urandom of=garbage bs=1024 count=1048576 while true; do md5sum garbage; done

example output:

99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
4dee7d74d6d8013291b48d955ea7af1f  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage
99e983a8f17f3258ddaff73bf622db47  garbage

dmesg has nothing interesting.

Any clues?

root@overo:~# cat /proc/version
Linux version 2.6.39 (neil@cumulus) (gcc version 4.3.3 (GCC) ) #1 Mon Aug 15 19:04:09 PDT 2011

Although I've also seen similar behavior on a recent 3.2 kernel using oe-core.

--Adam