Are IC sockets suitable for mains voltage?
I just grabbed one socket at random on Farnell UK, and got this drawing datasheet.  In it is this little bit of text:


  ELECTRICAL:
  
  
  Contact Resistance: 10 milliohms Max
  Contact Rating: 3 Amps
  Capacitance: 1.0pF per MIL-STD-202, Method 305
  Insulation Resistance: 5000 Ohms Min @ 500 VDC Per MIL-STD-1344, Method 3003.1
  Dielectric Withstanding Voltage: 1000 Volts (RMS) Per MIL-STD-1344, Method 3001.1
  


So I would conclude from that two things:


Datasheets with the relevant information in them are available if you look, and
Yes, that one would be OK for mains at low currents.


Besides, you'll have a resistor there too, right, which would mean you'd end up with maybe 2V across the LED anyway?

By the way, MIL-STD-1344A Method 3001.1 states:


  
  PURPOSE.  The purpose of this test is to prove that a given electrical connector or coaxial contacts can operate safely at its rated voltage and withstand momentary overpotentials due to switching, surges, and other similar phenomena. The dielectric withstanding voltage shall be established as 75 percent of the minimum breakdown voltage of the connector or coaxial contacts. It is suggested that the operating voltage of the connector or coaxial contacts be established as one-third of the dielectric withstanding voltage.
  


From that I can tell that the maximum normal operating voltage of that socket would be 333V RMS.

For clarification on the insulation resistance, Method 3003.1 states:


  
  PURPOSE. The purpose of this test is to establish the methods and procedures to be followed in determining the resistance offered by the insulation materials and the various seals of a connector or coaxial contacts to a direct current potential tending to produce a leakage of current through or on the surface of these members.
  


So it's intentionally making the insulation "break down" and testing the resistance during that catastrophic event.



Addendum: Don't sue me if I'm wrong.