OSX Lion Login loop problems: Is there a "verbose *login*" (NOT verbose *boot*)

Something happened to my Lion install last time I rebooted and now I can't login. I've got 4 users on my machine, which essentially boil down to my primary and admin account (no filevault), two friends who have loged in just a handful of times (w/filevault and have forgotten their passwords), and the guest account (which I also don't know the password for).

I've been to an Apple store and they had no idea what to do. They said reinstall Lion and do NOT recover from my last Time Machine backup as this might copy whatever fundamental problem is in place (instead manually recreate your stuff). That's a retardedly big hammer to use for what's probably a very simple problem.

My HDD checks out find. I can boot Single User Mode no problem. Verbose *boot* gets me to the login screen no problem. I've tried resetting my password, so now I have a new one that clearly authenticates properly, but that didn't fix the issue of a quick flash to a gray screen after I enter the password, then getting kicked back to the login screen itself. Note I don't have a recovery partition (I used to have a triple boot with Snow Leopard, Fedora, and Win7, but when I upgraded to Bootcamp proper for doing just a plain dual boot, however since I left the Linux partition the Lion installer wouldn't create a 4th partition for recovery). With that said, I have my OEM SL disk and the Lion "Internet Recovery" boot also works fine.

I'm looking for a general solution to my problem of not being able to login. I've read quite a bit online now and am dubious anyone will have a simple answer, so I figure the next smart step is to figure out what the heck is going on after I properly authenticate, which means, "find the login logs" (yes, I'm assuming those either exist or can be forced into existence, hence the title of this question).