It's a seamless transition (other than maybe your TV reporting the different viewing mode.) For all non-1080p events your TV will always be in 1080i (or 720p.) You should only see the popup from your TV when viewing 1080p events.
If you really want the nitty-gritty on EDID, on your Hopper hit YELLOW - BLUE - 3 - HDMI and scroll away

. In the "info" field if you scroll down, it will tell you the "Supported formats" which should include 1080p if your TV is 1080p-capable (which it obviously is since you watched 1080p.)
For anyone else that cares, on the ViP it's MENU - 6 - 3 - Analysis - HDMI (IIRC, I'll have to check later.)