I'd recommend bypassing the receiver and connect the Roku directly to your display. The factory default is supposed to be "Component 480i" (mode 1) which should work on about anything that has a component input. You could also try hooking up the composite and/or Svideo outputs, though Roku recommends only having one output connected. Once you have the Roku then configured for the correct mode you want via the Built-In Setup menu(you don't say what plasma you have or if it's true HD, most plasma's aren't unless you just recently bought it) then try going through the receiver.
Also, check your user manual. There are more detailed instructions how to configure the output mode using just the front panel buttons and watching how many times the power LED "blinks". This is what I had to do since when I first set mine up I didn't have a component display.
It also looks like you could telnet in (if you know the IP) and manually set the output mode in the /etc/rokucascade/settings directory by editing the 'deschutes-graphics-driver' file. But I don't know all the options for that file.
HD-1500 RevB #1 (media room)
Mitsubishi WD-62528 62" LCD RPTV
Kenwood VR-407 receiver w/PSB Alpha's
PS/3 60GB (now 250GB) w/750GB Seagate ext
HD-1000 RevB #2 (bedroom)
Visco 32" LCD
AR 2.1 audio system