Marshmallow has been out for about six months now, and there are still some very recent phones that don't have the update. For example, the T-Mobile Galaxy S6 and S6 Edge... until today. These devices are finally getting the Marshmallow OTA. Sprint and Verizon both rolled their GS6 updates out recently, leaving AT&T as the sole holdout. Read More
The next version number for Android is 6.0! We've confirmed the new version number in the official Android SDK, which was just updated moments ago to add the new platform. As for the name, we know it's Marshmallow, as per the statue which was just unveiled at Google HQ in Mountain View.
It also looks like the third developer preview is here alongside the SDK. Google says preview three will be the final Android 6.0 developer preview.
If you want more proof, here's Developer Preview 3 running on a Nexus 6.