Valve confirmed last night that the beta for the Steam in-home streaming had begun, saying that an undefined number of invitations had been sent out to users. Valve stated that it was just developer partners receiving the invites, but general steam users have managed to get in as well.
There are plenty of users discussing their thoughts on the service through the community forums, with some stating that there were issues and others stating that it is run smoothly. Remember that this is only a beta though so there are bound to be mixed reports coming in.
Valve has said that anyone who is receiving poor performance or graphical glitches, should update their graphics drivers to the latest version.
The in-home streaming service is free and allows users to play their Steam library on another computer which is connected to the network. There is no word as of yet whether or not it will open up to external streaming.