Concerning whether recovery is possible, I guess some of the questions are:
- How well does sdelete zero out the appropriate disk sectors? Does it ever miss something?
- Is there another copy of the file written somewhere else on disk, by either the OS or the application? Microsoft Office for example, writes temporary copies of files you are working on to the drive, and deletes (but doesn't overwrite) them once you finish your session.
- Is it possible to use a hardware interface to the drive, to read the data that has bled out between tracks on the disk. The ability to do this may relate to how many overwrite passes are made (which seems to be configurable with the -p switch in sdelete). There's a post about reading disk contents in this way on the sansforensic blog by Dr Craig Wright from a few months back if you're interested.
Post back if you give this a try, Id be interested in the result.