Sacado de Xboxhacker:
In good news for DivX player authors out there, a method of preventing the Xbox from rebooting when you eject the disk was found. There is no known way to *turn off* this mechanism, but it can be stopped from turning on in the first place. Using Visual Studio, go to "Xbox Image" in Project Options and add the following switch: /testmediatypes:0x80000007 . The Xbox will no longer reboot in this instance. It is the presence of the 80000000 bit that enables this.
Basically, if the 0x80000000 bit of an XBE's media type is set, the kernel doesn't turn on reset-on-eject mode.