Kris,
You mean a Hoffman-harpoon?
I don't think it's very practical for a PSK or EDC. But for a ship ditch-kit, maybe...
HTH
I know a number of people who use a Hoffman Harpoon in their PSK, BOB bags and as a necker. Practicality is not a problem for those that have a harpoon in their kits. With proper usage, I find it a nice addition to my gear.
TOPS has a larger version out now, and a mini Harpoon is coming out 3/24/08.