I do not thing they will make hard-drives obsolete. In order to do that, it would need to be:

1. Fast enough to give information to programs in time to keep up with the CPU
2. Able to write, rewrite, erase, and move information. It has to do it fast enough as well
3. Last more then a couple years...
4. Be used with minimal amount of energy, as hard-drives are used in many battery powered devices.