/sbin/sfdisk is provided by busybox, so if you're not making a typo and you're definitely running it as root in Maemo then you've got a bigger problem on your hands. What does Code: ls -la /sbin/sfdisk say?
ls -la /sbin/sfdisk
-rwxr-xr-x 1 root root 45156 Mar 12 2010/sbin/sfdisk