What solution do you use to bring a udp, rtmp, ... stream back into the hd-sdi format As a small independent TV Station we cooperate with festivals and congresses which want to broadcast their content live. Most of the times these partners send me a webstream. 

Currently we have a very basic solution in place
A machine with a videocard which is capable of extended desktop is used to play the stream via VLC and then send out the signal via the Videocards SDI port. 

What is your solution?
While I am pretty sure there are a few stand alone devices that can perform this process, they are effectively doing the same thing as you are doing by playing it back through the HD-SDI output of a computer.  RTMP is a very different type of signal than SDI and requires fairly complex translation to get it back to SDI.