If you had searched the forum before posting you would have seen you are the 4th person in about 4 days to report this, all with DirecTV. It seems your provider has suddenly implemented HDCP to prevent you recording (and slinging) HD content. The solution is to ditch the HDMI cable and feed your TV from the component OUT on the back of the Slingbox.
I have had my slingbox 350 connected to my TV for almost 2 years and I have never had an issue. I have been watching shows on my ipad traveling all week, now yesterday that message comes up. Does anyone know why out of the blue this would happen? I have not changed any connections to make this happen. I have direct TV box connected to TV via HDMI and connected to slingbox via component cables.