Quoted from Xbox-Scene
>> New games like MLB 2009 and Tom Clancy's H.A.W.X. don't boot from recordable discs on Xbox 360s with modified stealth firmwares.
The reason in short: these games include the NXE dashboard update inside the video partition of the disc. The newer 'stealth' iXtreme firmwares check the crc of the video partition and if it doesn't match 'wave1' or 'wave2' it will refuse to boot the disc. This new video partition that includes the NXE dashboard upgrade is 'wave 3' and support will have to be added for it in the iXtreme firmwares.
Older non-stealth firmwares do not check the video partition, so these will probably boot the discs, but it's unsafe (alias stupid) to use these firmwares on a Xbox360 you also want to connect on LIVE, as it can/will get you banned.
C4eva and his team are already working on a new iXtreme version for all drive versions. iXtreme v1.51 will probably include full 'wave3' support (including for Hitachi drives). v1.6 is planned for later with 0800 dumping for all drives as well! There are no deadlines for the release, it'll be out when it's ready and tested, so please don't ask ... it shouldn't take too long.