^I dunno andy, seems like Luffy was the more obvious threat, becoz firstly, luffy's closer to ace in terms of positioning, and secondly (more importantly), he is the guy who does the craziest things to drive sengoku crazy. The defeat of croc, the fall of enies lobby, the defeat of moria, and the tenryubito affair... All causes headaches for him. Even if the new world pirates are stronger, most dont have the guts to even try this sort of stuff. No wonder sengoku would be talking about him, and kizaru, since he would be like "WoE is he here now?" considering what Kuma did.
Though I dont know about Akainu's reaction, maybe he is an anti-virus software?
EDIT: Okay, a somewhat logical explanation of the drop-in, though I liked the death-wink theory better... dunno why
Crusader, u are most probably right in this case, probably Akainu will melt the ice and sacrifice his comrades to defeat the enemy. However, now that the joker card has been played (Jimbei), this could get interesting
EDIT 2: Some will definitely reappear in the future though, however the spotlight's on luffy. Now is the time to set the target for luffy. If he wants to be the pirate king (we noe he does), he would have to beat Whitebeard. Its all luffy's logic. Just like so many times be4, when he says, "So if I beat this guy, then..."