If one could obtain a private key from an address, Bitcoin and all public-key-cryptography was doomed.

The process is:

private key -> public key -> address

Both arrows are irreversible.

That said, bitcoin 0.6 will have an RPC call by which you can ask bitcoin the private key corresponding to a given address, given that it is your own.

Thanks for responding, um im just trying to do paper bitcoins like others have done, maybe i worded it wrong, but i just would like to know how they did it as i would like to do the same.