I just wanted to follow-up. I had a ticket open with Slingbox support. Here is the reply that they gave:
"I am sending this email to inform you that although the Roku supports 1080p for a resolution, the SlingPlayer for Mobile devices is limited to 720p or 960/544. The reason this information is important is because when you cast from you mobile device, the Roku mirrors what it receives from the application."
A few observations that I had:
My resolution is not limited to 960/544. The max is 1280x720. I'm not sure why they mentioned 960.
For the Roku/mobile app connection, I kind of understand that. However, from what I've read, I thought there was a handshake from the mobile app to the Roku. Once the streaming is occurring on the Roku, I thought the mobile app was irrelevant. Maybe it's during the initial handshake?
Anyway... I guess the bottom line is... anyone hoping for 1080p (even on a LAN) is out of luck. I've put an enhancement request in, but I'm not sure if that type of thing is being done.