If you have a custom ROM or SpeedMod or any I9000 based kernel, you should download ODIN 1-click, MASTER CLEAR, then flash the AT&T ROM. After that, you can use bay_wolf's AIO Rogers toolbox to get back to Rogers Froyo. I then used Kies to get my Froyo on, then flashed the Rogers Voodoo Kernel. After that I rooted and reinstalled my Titanium Backup.
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Did you end up having the AT&T boot screen after flashing back to Rogers Froyo through that method?