We don't have a lot of information yet (it just started, but we try to warn you all as early as possible), but it looks like Microsoft started a new wave of bans against modified DVD firmwares or DVD+/-R DL discs.
Several users on our forums and on IRC report their Xbox 360 has been banned from LIVE today. We get reports of Toshiba-Samsung and Hitachi-LG drives banned with both Xtreme and even the newer iXtreme firmwares. However we also see many reports of people who are currently on LIVE and have no been banned (yet). No reports of banned BenQ drives yet (but few of these drives have been flashed yet).
We don't know yet if Microsoft is detecting the modified firmware or the discs (or maybe only with a specific game?).
If you have the 'Z Code' 8015-190D it means you are banned. As always Microsoft bans the console serial from LIVE, not your LIVE account.
If your console is banned or if you have been playing on LIVE today with modified FW without getting banned, report you findings in this thread (include at least your live status, full drive model (swapped drive?, spoofing?) and firmware version used).
So until we have more info we HIGHLY suggest you don't log in to Xbox LIVE and don't play any games from DVD+/-R DL (even offline, incase MS flags something) if you have a flashed DVD drive.