I'll take a shot at this one.

Some surround programs aren't in true 5.1 surround. They are broadcast in what is sometimes called "simulated surround sound." This process supposedly expands the image of the program sound to better "surround" the viewer with sound.

Now, when you select "Surround" on your 1050, the receiver searches for the input. If it sees a Dolby Digital stream, or a DTS stream for that matter, it processes that information and provides you with true 5.1 surround sound. If it sees just a two-channel input, it fakes the surround. The 1050 does this by using Prologic technology.

So here's my conclusion. Unless the program specifically says it's in Dolby Digital 5.1 surround sound, it's being broadcast to you only in stereo. Thus, the ProLogic symbol shows up on your 1050 since it is in "Surround" mode and it needs to fake the surround sound for you. Watch the receiver when you go into a DVD menu for another instance of when this happens.