There is currently no easy way to do this. There should really be a preference somewhere to set the default front row screen. This should have been a feature in front row 1, but it wasn't and still isn't. I maintain that apple deliberately makes front row difficult to use to sell more Apple TVs, which is a shame.
Anyway, the bug you are experiencing goes like this:
You have two displays, iMac and TV. You want iMac to be your primary display always, and front row to show up on your TV -- the secondary display. You don't want to mirror your displays for obvious reasons.
This is impossible.
Front row does not fully quit when you exit back to the OS. It stays running as a background process. When it starts up cold it asks, who is the primary monitor? Say it's your TV, it saves that information and starts up on the TV. It also says I will now appear on the TV no matter who the primary display is. So after exiting front row, but not killing the background process, you switch your displays to make the iMac the primary. Then you enter back into front row and it magically appears on your TV because it remembers that that is where it first started up. If you go in the terminal or activity monitor and kill the front row process, and then restart it, front row will appear on the iMac.
This could have been a workable situation had apple not decided to also make the front row process self terminate after a pre determined time of inactivity. You could imagine first starting front row on the TV and then switching the iMac back to the primary display, and as long as you did not reboot, front row would always appear on the TV. Not the case.
The only solution is to first kill the front row process if it is already running, switch the TV to the primary display, and start front row.
I developed a good way of doing this using a combination of applescript, a utility called cscreen, X-keys (a hotkey manger), and a bluetooth cellphone as a remote. I believe this is the only completely "from the couch" solution.
here is how its done:
1. download cscreen (a utility to swap the primary/secondary monitor)
http://forums.macosxhints.com/attachment.php?attachmentid=1416&d=1156306718
place cscreen in your applications folder
2. write an applescript to swap the screens and kill the front row process
(i'll paste some code later tonight, i don't have it at work)
3. set x-keys or some 3rd party remote software to run the applescript
4. start front row
this procedure has served me well for a year now. using a bluetooth remote is actually more useful here than the apple remote because it can be used to launch the applescript. if you don't have one, you will at least have a one click solution.
oh the things we have to come up with to use our computers...
goodluck