Fox News Chicago is reporting that Senate Candidate No. 5 is Jesse Jackson, Jr., which lines up with TPM's guess and that of our own Mick Dumke. Here's why candidate 5's identity is a big deal. Update: ABC News is reporting the same thing (via), and he responded with something of a non-denial.
Then again, FNC said Rahm leaked the Senate pay-for-play request, which he denies, so YMMV. Here's JJJr's statement.