If you're a modder, and maybe even if you aren't, you could be in danger of turning your 360 into a brick, courtesy of Microsoft's latest firmware update on the 360. Tread with caution and read the link below.
http://xbox360.qj.net/Is-Microsoft-striking-back-with-a-bricker-/pg/49/aid/71274
For what it's worth, a coworker of mine got the "3 lights of death" on her hubby's xbox 360 immediately upon rebooting after the update finished. Dunno if she was modded, or just had some suspect hardware or if its all just an unfortunate coincidence. Either way, you've been warned.
edit: here's yet another site link, with the same basic story: http://www.xbox360fanboy.com/2006/11/01/update-brick-your-360-speak-up/