Just an observation for Dan and the team as opposed to a bug.
I can fly around an airport and see all of the scenery in monitor mode, but when in FlyInside some of that scenery will mysteriously become invisible. An example would be a private jet and some other aircraft and scenery at Boscombe Down in the UK ( using ORBX EU England ), which all become invisible when using the Rift.
Not sure what might cause that and I don't have an immediate answer myself but though it worth mentioning in case it's a general bug and not something specific to my machine...