I captured ADS-B packets from a U-2 in the LAX area (using an RTL-SDR dongle) on 2013-06-18. If you want to see them for yourself, the decoded packets (hex) are at https://dl.dropboxusercontent.com/u/7443278/Mode%20S%20Captu... (along with other aircraft).

The U-2 I picked up is ICAO ae094b, registration 68-10336[1]. You can decode the packet information using dump1090[2]:

  ./dump1090 --net-only > out.txt

Then in another terminal:

  cat mode-s-capture-2013-06-18.raw | nc localhost 30001

Then in out.text, look for "ae094b". You'll see packets like

  *0206042b1365a5;
  CRC: ae094b (ok)
  DF 0: Short Air-Air Surveillance.
    VS             : Airborne
    CC             : 1
    SL             : 0
    Altitude       : 60000 feet
    ICAO Address   : ae094b

[1] http://www.airliners.net/search/photo.search?regsearch=68-10... [2] https://github.com/MalcolmRobb/dump1090