Even more Ussop actually took command at the last second on the bridge: It was him who ordered Robin to toss Luffy into the sea and told everyone to jump - he even argued against Zoro.
Great point! Soge consulted Zoro on the bridge before the battle and followed Zoro's orders then – but he went against Zoro's orders a few minutes later to save Luffy. I think this happened for two reasons : (1) Usopp had a plan for how to save Luffy and Zoro didn't and (2) Usopp was able to prove to the others that Zoro was emotionally unfit to command them at the time.
We know that Zoro didn't have a plan for how to save Luffy because of what he said ("Luffy's in trouble! If we could just get him over here ..."), and we also know that his inability to think of a plan affected him emotionally (look at Zoro's facial expression when Usopp grabs his shoulder -- we haven't seen that sort of desperation in Zoro's eyes since Baratie!) Many AP folk picked up that vibe and asked why Zoro seemed so "tense". This was why : because he couldn't think of a plan, and his inability to think up a plan meant -- in his mind at least -- that Luffy would die.
http://groups.msn.com/onepiecemangav-2/428.msnw?action=ShowPhoto&PhotoID=9268
But Usopp did the right thing : overriding Zoro's authority temporarily. He may have subverted the hierarchy, but in doing so, he saved his captain.
[sorry for the digression! I'll talk about the current chapter from now on.
Oh, and Rootbeer – thanks for correcting me on both things. I can tell that I've got to reread the Drum arc again soon, as a lot of it will echo in this chapter.]