Recently, while reportinga story, I discovered that looking up the average wait times at airport security lines is about as frustrating as actually waiting in line at a checkpoint.
The TSA used to have a Wait Time Calculator for terminals across the country, but that online service is shuttered through 2009, at least. That left sites like FlightStats without current data for even the major airports. (For instance, I tried searching for data on New York City and Chicago airports over the past week, and wait-time data has been unavailable.)
iPhone users can download apps (of course) that include security wait times. TSAwait, $1 in the iTunes app store, was relying on the TSA wait times calculator. They still use the most recent information from the TSA at 400 airports and more than 700 checkpoints, but they're working off old data that has probably changed.
Travelocity also publishes an iPhone-optimized site that has average security wait-times. The data is gathered from a yearly "Thanksgiving Task Force" survey done at the 10 of the U.S.'s busiest airports. But the stats aren't available for every airport, and it's only valid around the holidays.
Orbitz has a Traveler Update section, with some average wait times at larger airports—but, again, it's not complete research for every airport.
Your best bet might be to check the website of the airports you'll be using. Atlanta's international airport, for instance, has updated security wait times on its homepage. (As an aside, the airport also has a Facebook page—who knew?)
If you're a type-A traveler like me, timely, thorough info is useful (and reassuring!). It's also nice to learn in advance, for example, that a particular checkpoint in terminal X sees far less action than other checkpoints (don't you love feeling like you've beat the system?) But until the TSA gets their site back up and running, we'll have to look a bunch of places for just one set of information—a frustrating prospect for any traveler.