As I've had my xbox 360 since the dawn of time (and the beginning of flashed consoles) I've yet to be banned. There's many steps you can take to insure that you won't be banned for playing a downloaded xbox 360 game, such as stealth and verification patching.
Microsoft has developed ways to detect some flashed firmware, but from what I've read and found out, thats only for the older versions of iXtreme. The latest are currently undetectable (mostly, from what I hear. Some people have been banned, but thats been from improper patching, burning & advertising the fact that they have a modded console.)
If you need help with it, I can teamview/skype/MSN if you want.
Let me know.