I have a similar problem.
I have an intel imac with xp installed. Im having real trouble burning backups and getting them too boot. I have done a lot of modding in the past, but this is my first attempt with the 360.
Anyway in my house there is me and my housemate. I ahve the iMac, he has an old xp pc. His pc has a dvd burner but it is single layer, therefore cant burn the backups. However we can use his pc to rip the xbox games (and flash the 360 firmware, though a friend did mine as at the time didnt realise we had the right connections on housemates pc, heh)
Long story short though, only duel layer burner we have is the iMac. I know its possible after searching around to burn 360 backups with bootcamp on a mac, but neither of my 2 attempts have worked. Im using traxdata discs which is what my friend recommended, but I know its possible that they may not be right for my burner.
When I put the backups in the 360 though I dont get the message "to play this put in a 60 system...etc". It says "eject disc" on the tray icon on the dash, and nothing else. its bizzare. I have no idea where im going wrong.
Im using clone cd in xp on my mac, and treating it as a windows pc.
Im in the process of burning an ttempt at my second backup of marvel ultimate allience right now though to check if I jsut ripped the original image incorrectly, and will let you know hw it goes.