dexter222 said:
Thanks for such a quick and detailed answer... but was that a "yes" or a "no"?
You can always get banned with a custom Firmware, that's the risk you're taking, even when only playing the retail games online. One simple Update, and you could be gone from XBL. Right now, if you aren't on LT or LT+, you are probably already flagged (Flag=Banned when Microsoft starts a new Banwave, nothing you can do against this). Same goes for not verified games, always verify them with ABGX360, I would recommend to only play games which are 100% verified, otherwise you have a higher chance to be banned.
The AP2.5-Checks are also really nasty, right now they don't seem to be used that much (only for a handfull of games), but it should be possible for Microsoft to update them via an Game-Update (they also can update them via Dash-Updates). Right now, LT+ should protect you from a ban/flag if the checks fail.
Tl;Dr: If you always update to the latest Firmware, don't apply Updates without searching first if they are safe, and always verify your games with ABGX360 (and only play the 100% verified games), you are as safe as possible. That doesn't mean that Microsoft won't/can't ban you though. Nothing is ever 100%-XBL safe, only Retail-Games and a retail Firmware.
The tool
XVal 2.0 should be able to tell you if you're already flagged or not, btw.