Amusingly enough, I have a device that I could probably use the 'unusable' drives in. What they do when they modify them is remove the profile that lets them act like a memory card - they're left in "IDE" only mode. You could probably hook them up to a IDE controller and use them that way. Most portable devices that try and use the card can only use the memory card profile, so *poof* - 95% of the portable market that uses CF cards can no longer rip them out of the Mini/Creative Labs/Whatever mp3 players.
I've got a Fujitsu Stylistic 1000 - a tablet with a 486-100 processor and 16mb ram. It has 2 sets of PCMCIA cardslots - one is a normal PCMCIA Type II/III which you normally see on a laptop, and the other is a PCMCIA Type III which is for IDE only devices. The hard drive for the tablet is a PCMCIA Type III drive (I've got a 340 mb and a 40 mb drive). THe 40mb one is a weird Maxtor drive without a memory card profile, so it only works in the IDE only slot.