TPMMuckraker, with the help of GOP Progress and porkbusters.org (who originally started the tally), has narrowed the list of possibilities of the "secret hold" on S. 2590 in the Senate to five candidates (in fact, in the time I took writing this post, the list narrowed from eight to five):

Robert Byrd (D - WV) Mike Crapo (R - ID) Judd Gregg (R - NH) Orrin Hatch (R - UT) Ted Stevens (R - AK)

Of those five, two are former Chairman of the powerful Senate Appropriations Committee and could have more of a reason to be sheepish about disclosure of how the government spends its money than the others (they are long-time Sens. Byrd and Stevens btw). Keep your eye on TPMMuckraker's running tally of denials of the hold as the secret Senator is unmasked - maybe sometime later today!