How likely is it that it will be implemented?
I would like to have this feature too, to be able to verify the integrity of my downloaded files or files in general.
Btw: I saw this on the feature page of GnuPG: Supports ElGamal, DSA, RSA, AES, 3DES, Blowfish, Twofish, CAST5, MD5, SHA-1, RIPE-MD-160 and TIGER.
But I wasn't able to figure out, how to use GnuPG to calculate the md5 sum for a single file.