So in Into Darkness we see Kirk been busted down to first officer by Pike, now did that mean we saw him been demoted in rank to Commander? Now I now he says to Spock not to call him Captain and in response Spock calls him Commander instead on the way to the big meeting. Did his uniform reflect this change in rank, I couldn't tell? Was this the first time we've seen a Commander Kirk?
IIRC, Kirk does wear the insignia of a Commander when they're at the all-hands meeting with Marcus. It would have been the first time we ever see Kirk wear the insignia of any rank lower than Captain. (For most of ST XI, Kirk is a Lieutenant, but does not wear a full Starfleet uniform.)
I just checked the scene in question on the DVD, and Kirk is still wearing captain's insignia at the meeting.
Jim Kirk was promoted at the end of the first movie to the rank of Jim Kirk. This rank is inalterable due to narrative necessity. Trying to square any of this with how a normal human organization would handle ranks is a one-way ticket to the puzzle factory.
Thanks for checking Avro Arrow, so it seems he didn't have time for a uniform change or he was still Captain as Spock believed. But Kirk specifically corrected him and seemed to be ok when Spock called him Commander. Was Spock wearing the right insignia in that scene? Guess it's all moot as Marcus obviously reversed the demotion, but it'd be nice to get confirmation that the demotion occurred!
That he is demoted to executive officer of Enterprise is beyond doubt. Although he acknowledges that his 'official' rank in that position is Commander, as Pike has been made Captain of the Enterprise again, maybe that doesn't necessarily mean that his actual rank takes a hit. Maybe he can still technically be a Captain and yet also be an XO/Commander. His "spoken rank" is really only reflective of his position aboard the ship, at least for the 20 seconds that Pike has resumed the Captaincy anyway.
The Abramsverse seems a bit cavalier about rank, though (and not just Kirk jumping straight from Lt. to Captain. What about Cupcake, for example?), so it's not really surprising if somebody's not wearing the right insignia.
I'm pretty sure he's still in Captain's dress uniform, with three stripes, four pips and the TMP-style insignia set in a circle. A commander would have two stripes, three pips and a plain arrowhead (and Admiral a white uniform front and gold insignia set in a circle) Since Kirk presumably walked right out of Starfleet after being bumped back to the academy, then went straight from the bar to the emergency meeting, he didn't have time to requisition a new uniform.
Spock was wearing a full commander's insignia. As Hartzilla2007 mentioned, Spock wasn't actually demoted, so this insignia was correct. Pike was still wearing admiral's insignia at the meeting, too. We really don't know if they demoted Pike to captain, or were just going to have him command the Enterprise but retain his admiral rank. They technically could take away Kirk's command and still leave him ranked as a captain. But then why would Kirk correct Spock when Spock called him "captain"? Interestingly, Spock is the only one that calls Kirk "Commander", and only after Kirk tells him to not call him captain. Pike and Marcus both speak to Kirk during the meeting, but neither refer to his rank. Well, he went somewhere between the bar and the meeting, because he was wearing civilian clothes in the bar. (Pike specifically told him to "suit up".) But yeah, you're right that he probably just grabbed the uniform he had available and didn't have time to requisition a new one. Yeah, this is true.
I'm not sure how closely Starfleet follows naval tradition, but from how I see it, Spock is not totally incorrect about calling Kirk 'Commander'. Traditionally the commanding officer of a vessel is addressed as "captain" regardless of actual rank held though a flag officer in the chair may retain his/her normal title of address. Since the ship's master is THE Captain, captains from other branches on board are verbally called by the next highest rank (Army/Air Force/Marine Captain -> Major) to avoid confusing the crew. Kirk lost the command position, but I don't recall him losing rank. As he still holds the rank of Captain, he cannot be "promoted" as Commodore/Rear Admiral is a flag officer rank. Spock had no evidence that an Admiral such as Pike was assuming command rather than another captain, so the only other choice from his perspective was a verbal 'demotion' to Commander.
That was kinda my interpretation. He has not been stripped in rank per se but only in his position on the Enterprise. If "Admiral" Pike is in command then *he* is the Captain - which means that Kirk as the executive officer is the Commander regardless of his actual official rank being Captain. Spock correctly calls him Captain but Kirk corrects him by underlining that he is really a Commander now to all extents and purposes.