Zero knowledge proof for value of a hash function Is there a zero knowledge proof which demonstrates that Peggy knows a value v whose hash-function is w?

In my understanding of the general theorems on zero-k there EXISTS such a function if the has-function has polynomial time complexity. However, I would like to know how such the protocol would look like for the case of SHA-256. My problem is, that the usual proof requires me to reduce the fact of having a certain SHA-256 to Graph 3 Coloring, for example, and I do not see how this could be done practically.
In the rump session of Crypto 98, Hal Finney provided a talk which might be of your interest. The title is "A zero-knowledge proof of possession of a pre-image of a SHA-1 hash." I have not found any transcripts., but the video is available on both YouTube and Google Videos. (the videos are no longer available; see the comments below.) The video is now available from my Google Drive.