Excellent timing, I've just announced on a mailing-list my first public AMI. So I'll have to check tomorrow if I made mistakes on the way :) (I'm pretty sure I haven't left anything confidential, but there might be some cruft with my keys…)
libguestfs[1] that I helped to write provides some solutions for this, including access to secure scrub utilities[2] and this script[3] for removing "empty" space in a disk image.
