Ok. 

A few years ago a backed up a whole bunch of my photos by tarballing
them up and encrypting them with Blowfish 128 using GPG. Since that
time, I've lost the unencrypted versions, but found a copy of the
encrypted tarball. But of course, I've forgotten the password.

Can anyone suggest a possible method for recovery? I'd even settle for
a brute force dictionary based attack (on my own file...) if I could
find a program to do it (and leave it running for a few years...). I'm
just after pointers in the right direction, if there is one.

Please note an answer of "You're stupid..." won't cut it cause I know that already.

Assuming you are prepared for other people to have access to the files
your best plan may be to promote this as a distributed cracking
challenge, along with anything you do remember about your passphease
(such as the length it might be and whether it is a combination of
words).

With a prize and some code you might attract a reasonable following
using only cheap publicity.  I suggest code derived from GPG because
driving an entire GPG process repeatedly from an external program is
wasteful of those precious CPU cycles.

I think you will find that you are pretty much without any hope here. 
Your best bet is to try to remember all your old passwords, type them
out, and then go through the list one by one until one works.  If you
can't do that, well, you have a chance if you create a wordlist that
matches your typical style for picking passwords.  If you pick
passwords like I do, though, you are pretty much SOL.