Before we start, no it's not Blu-Ray and it's been confirmed. Anyway, confirmed by Digital Foundry, Microsoft is set to test out a new disc format for people in North America. Unfortunately, Microsoft has closed off the sign-ups for eligible testers really quick. As for being a tester, Microsoft will be sending copies of Halo: Reach, which most people presume actually is the updated disc format.
As far as details go on the disc, it is said to give developers 1GB added to the dual-layered DVD, so that more data can obviously be added. Many sites, including myself, are assuming that this may be a newer way to eventually catch people who have pirated games and modded their Xbox 360 consoles.