This has finally been confirmed, in the season 11 episode "Don't Call me
Chuck: Oh, I'm sorry, I always forget: people can't see me unless I want them to see me. It's very confusing. Here: [presents Metatron with a pair of Wayfarer sunglasses] visual aid. Put these on, they'll help. Go on, just do it; it's a whole thing.
[Metatron puts on the glasses. Chuck taps his finger and is replaced with a bliding flash of light as a heavenly chorus sings]
Supernatural Season 11 Episode 20: "Don't Call me Shurley"
The Road So Far
This was one of the more popular fan theories.
At a 2011 JusInBelloCon panel, Rob Benedict (who played Chuck/Carver) pretty much confirmed that his character was God:
Audience member: I was wondering, if you are the God for real, on Supernatural, how does it feel?
Benedict: It feels awesome. To be all-knowing, it's awesome. No, you know, the funny thing is: I didn't know that I was God, and then they sent me the script, and I read it, and I thought "Oh, I'm wearing white; alright." And then I get up there and the crew - the people who work on the show - are all going "So, hey; God, huh?" And I said "Am I? [undecipherable] God? Okay." And then Eric [Kripke] called me after the episode, and he's like "So how did it feel to be God?"
However, in a 2010 interview following "Swan Song" airing, producer Sera Gamble refused to confirm or deny:
When I talked to you guys at Comic-Con back in the mists of time, [then-showrunner Eric] Kripke mentioned that we'd be meeting God this season. Does that mean we actually did meet God, and now we're left guessing about who it was (Chuck, Crowley, Joshua, etc.)? Or did you guys decide to leave God out of the picture?
I love a good God debate, so it's nice to hear we got one going this season. We purposely left a bit of room for interpretation. Although many of your readers probably just read that sentence and rolled their eyes because they feel like we made it all very obvious by the end.
Nobody thought it was obvious! Do you mean Chuck is the obvious answer, or that you really did leave it open?
Don't ask me to squish the God debate! How many TV shows can say their fans are talking about THAT stuff?