I just flew back from Laguardia a couple weeks ago with a coiled Kryptonite cable I purchased in Brooklyn in my carry-on bag. No problem whatever.
When I traveled ORD -> LGA, I was asked to remove the lock from my bag for inspection by TSA. They let me go on my merry way.
However, on the return journey, the LGA TSA clowns said I could either check the lock or abandon it. I had little interest in going back to the ticket counter to check a single U-Lock, so it is in a pile of nail clippers and lighters in NYC.
Your Mileage May Vary, but when in doubt, just check it.
It just so happens I ran run into problems with this a few years back. Flew with a Ulock in my carry on luggage out of Chicago a few years back to the Twin Cities. It was the weekend "anniversary" of 9/11 in 2007 or 2008 if memory serves. I don't remember if I checked in in Chicago, I don't think so. No problems in Chicago going through security.
On the return trip (flying out on 9/11) I tried to bring the lock through security at the Twin Cities and I was flagged and questioned. Seems they thought it looked like a dangerous weapon, even after I explained its (security-related) function. How's that irony for you? They did let me go with a warning not to bring it through security again.
Given that TSA is all over the map on this, my advice: check you bag/lock in, even if it costs more. TSA will be jumpy enough this weekend I'm sure, don't give them any excuses to flag you.
I had to abandoned a combination pedal/15mm wrench because it was a "tool longer than 7 inches." Did they measure it? No.
Did I pay $25 to check the bag? No.
I tossed the tool - not a hard decision.
The TSA website says you can bring a "bike lock". "U-lock" was not found in the search.
I'm going to try this tomorrow.
Yes, I've had to abandon a pedal wrench, but it was at least 10 inches long. Now I don't travel with my main cycling messenger bag and haven't lost anything else.
I would check the bag and avoid the annoyance all together.