Over on our forums, user sysrun has posted about his fork of the rtl_fm command line tool. His fork allows retuning of the rtl_fm program via UDP without the need to restart the program. In the future he hopes to support UDP PCM streaming. He writes

rtl_udp is a copy of rtl_fm with a special feature: It opens a udp control port (currently fixed to 6020) which takes commands like changing the frequency or mode. No need to restart 

Credits, Idea & original Code by olgierd (http://qi.reddit.com/user/olgierd)

Usage:
python script udpclient.py for easy operation included.

possible commands:

• freq (./udpclient.py freq 101900000)
• mode (./udpclient.py mode 0 (for fm))
  0 = FM
  1 = AM
  2 = USB
  3 = LSB
• squelch (./udpclient.py squelch 0)
  0 = OFF
  n = Value
• gain (./udpclient.py agc auto)
  auto = Automatic
  n = Gainvalue; 195 = 19.5db
• agc (./udpclient.py agc 1)
  0 = OFF
  1 = ON