Yep, Only way to REALLY erase stuff off a harddrive is to rearrange the drive itself! Magnetio! Check this link
http://www.bodybuilding.com/fun/par14.htm
Get a large piece of steel, a car battery and lots of metal wire. wrap the metal wire around the piece of steel and put one end on the positive and one end on the negative. Voila! SUPER powerful electromagnet!
BECAREFUL, wear THICK rubber gloves. The volts running through that are very strong and can potentially kill a person.
Possible attach one end to a switch, connecting battery -> switch -> metal wire and keep this on the side of your computer so if the feds come a knockin' simply flip the switch and EVERYTHING on your computer is GONE and to my knowledge cant be salvaged.
A PGP encrypted file can do wonders.