Yup, there is DRM in HoMM6.
It's also true that after you register the game online once, you can play the game offline, however:
1. you can't play/load your online saves
2. you can't use dynasty traits, weapons and pets (existing DLC introduced a pet that isn't included in the vanilla game) in offline games
3. you can't peek/use ingame conflux spots (which in most cases has hints and warnings if critical bugs are still unfixed on the current map)
4. features on your online conflux acc (
http://heroes-6-conflux.uk.ubi.com/) are utterly useless
The biggest negative issue of this always online although singleplayer DRM is that fan game mods are pretty much impossible to make.
The patch 1.6 be released in a week or two but it will not address any of this as there will be many bugfixes and balance changes. However that's not the last patch planned for the game. On Ubisoft official forum Limbic CEO has his own thread about game changes/patches/enhancements and he stated that there will be also a patch 1.7 in about a month after 1.6, but didn't specify what exactly will that one bring, it's possible always online DRM will be removed with it.
HoMM6 is a great game and I'm still having fun with it. It's not a masterpiece, but it's really well spent money and if they remove draconian DRM, what Drithius speculates will probably become real thing:
And I too hope they retroactively get rid of the DRM in Heroes VI. It would be funny if there were a noticeable spike in that game's sales the day that patch is released.