Capcom will release Resident Evil 6 on October 2, 2012, but PlayStation 3 and Xbox 360 users both will have a chance to give it a test drive starting on September 18, 2012 with the release of a playable demo (via Siliconera). On the Xbox 360 side, that date just applies to Xbox Live Gold subscribers; free Silver users will have to wait an additional week, with the demo arriving on September 25.
That is, of course, unless you nabbed yourself an Xbox 360 version of Dragon's Dogma back in May, since launch versions of the action/RPG came with a code offering early access to the RE6 demo. Those folks have had access to the demo, which includes co-op play pulled from each of the game's three campaigns, since July 3. You'll be able to check it out for yourself if you haven't already in two weeks from today.