jas0nuk recently released the source for PRXDecrypter, his nice little PSP decryption program. I took it upon myself to merge in the decryption keys found by coyotebean. Now, jas0nuk's nifty little program can decrypt the PSP Go files for FW 5.70, 6.10, and 6.20 :) thanks to jas0nuk for this wonderful little app, and to coyotebeans for the new keys.
comes complete with my modified source.
-- UPDATE --
Here's version 2.6a, which includes the new OFW 6.30 decryption method and keys from hrimfaxi.