The reason is threefold = 419 hits
The reasons are threefold = 597 hits
I've mostly heard people use that plural one. But today I came across the "the reason is threefold" (Yes, it's another thrilling day of copy editing).
Dictionaries don't address this issue directly. But, the way I read their definitions, it seems to make more sense to use it with the singular.
Webster's New World:
threefold adj. 1. having three parts. 2. having three times as much or as
many. adv. three times as much or as many.
Dictionary.com offers examples:
a threefold program
a threefold return on an investment
So does Webster's Revised Unabridged (via Dictionary.com)
A threefold cord is not quickly broken. -- Eccl. iv. 12
Seems to me that when someone says, "The reasons are threefold," they're using "threefold" as a synonym for "three." (Answer me these questions three.) But none of the dictionaries I checked treats them as synonyms.
I hadn't had much time to look at the news until late in the day, but this is for those of you who were interested in the politics of yesterday's post: The Center for Responsive Politics has released a report on how much money congress members got from banking/financial company donations in the years before a 1999 vote on removing some regulations. As also reported in the Los Angeles Times, the report demonstrates a clear correlation: Reps who got the most money from this industry were pretty much the same people who voted yes on the deregulation measure. Here's the link, where you'll also find a full list of names and dollar amounts: http://www.opensecrets.org/news/2008/09/money-and-votes-aligned-in-con.html
The Times also reported today the stuff that was on the Wall Street Journal's website yesterday about lobbying by banks, etc. to capitalize on the bailout plan.