I'm afraid that what you want to do is not possible.
You can come close by using EDS on the phone, but your computer will not be able to read the files directly. TrueCrypt will be able to decrypt the containers, but I think you might have some problems with handsets using MTP rather than USB Mass Storage.
Essentially what you want is for Android to support the "Secure" part of "Secure Digital". What you want to do is built into the SD card spec, where a password can be set on the card and can then only be read on that device.
For instance, the Samsung Focus WP7 handset locked SD cards to it, and these were not readable in other devices. Most devices simply saw the cards as unreadable, not even able to format them. Only Symbian handsets seemed to be able to recognise these cards were locked to a handset and were able to format these cards, but not read the data that was on them.