How do I invoke "su" from a terminal prompt?
The clue is in the owner/group id... 

$ ls -l /system/bin/su
-rwsr-xr-x root    app_101    26234 2012-07-09 15:00 su


Notice the group id is set to 'app_101', that should be 'root', but then again, you need the busybox binary (which is part of the SuperUser.apk binary, when installed, it copies the relevant said binary into /system/xbin) in order to be able to chown it.