RKCOJOHN: Thanks for the tip! I checked my Airport Extreme Base Station settings, and your suspicion was correct -- it was set to link-local instead having IPv6 automatic turned on.
So, I switched that over, and it didn't seem to make a difference. So then I completely reset the Airport -- "restore default settings" -- created a new network, and left IPv6 turned on for both the Airport and the iMac.
The good news: I now pass all of the usual IPv6 browser tests -- like www.ipv6.apple.com and test-ipv6.com -- which didn't use to work for me. So your tip on updating the Airport settings resulted in this nice perk!
The bad news: It had absolutely no effect on getting my iPhones to show up in iTunes on my iMac. iDeclare, iProbably just have to wait for Apple to issue another Mavericks update (like lkrupp notes above), because something is amiss here.
I don't know if it helps much, but I've filed a couple more tickets at Apple.com/Feedback about Mavericks causing havock with iPhones connecting to iTunes and various SMB network share issues. Fingers crossed.