Issuing '/nui roundmap' twice does reset the map back to being square, BUT, logging out or exiting the game resets the map back to being round. So you have to manually reset the map back to being square every time you switch to another character or re-log back into WoW. At least that's what's happening for me.
Obviously Blizzard changed something, as for some reason the state of the mini-map is not being saved between game sessions and always defaulting to the round mini-map.
I guess one of our resident nUI programming gurus needs to make a patch to make sure the state of the mini-map is saved in the nUI WTF files.
EDIT:
Well, a little more investigating showed that the problem I'm seeing is a combination of Blizzards release of 5.3 and an interaction between nUI Plus v5.07.32 and Carbonite 5.3.0-Alpha 1. I turn off Carbonite and nUI displays its default square mini-map, turn off nUI and I get the round Carbonite mini-map. At this point I don't know which of the two addons is the one trampling over the other, as this wasn't a problem before WoW patch 5.3 and the two addons co-existed with each other just fine. As addon problems go though, this is definitely a minor one.