The connectivity issues made me pissed. I decided to give it another go. Seeing as I'm pretty much done with single player, I'm really just in it for the multiplayer now.
It didn't work. I got dropped and left in an endless loading screen loop. I think I've found something that can be seen as a solution, though: I first enter Single Player. Pull up the menu and select Multiplayer. From here, simply choose Xbox Live Private, because Private seems to work most of the time. And that's it.
I'm stuck in private untill someone fixes this stupid-ass connectivity issue. I think my NAT is moderate and I'm too lazy to find out how to open it completely. I might as well do some achievement whoring in single player. There's at least one I haven't gotten yet.