... I think it may have been a brute-force attack.
Unlikely unless you're sloppy.
To prove the point, 10 bitcoin for each pass for any of these unsalted MD5s:
824cfad07c88261afb4dd3285627887a
73550477b12849b2a4dcd3b0db187415
3e567bcbb2aa5c28c47012b857bf6e48
3709fb6b0e1c0b26ff22a19ae92fd080
9133c451dd761d29943dcc653252e2fa
ff111d6144367b4abd99aa4321b0a618
8602188ef5a05a13afc59c51b395426c
da842aa7c84236d17a04098fa1273f2d
Have fun! ;P
EDIT: alphanumeric only. I'll pay legitimate finds or my name is mud!
Well they aren't in any rainbow tables, so they must be pretty long. Judging by the high reward on this, he probably used 15-20 characters. Enough that you might as well keep your computers mining bitcoins, it could be months even for a very powerful group of computers.