Hello everybody, I'll try to keep this short and topical. I'm looking for someone to point me in the right direction, not for help diagnosing the problem, necessarily.
I was careless with /usr and /usr/share and made some recursive changes with chmod and chgrp: adding the setuid and setgid bits to /usr, changed /usr group to adm, then making /usr/share writable by group.
This, of course, made sudo and mdm fail. I fixed those, but now I have problems with various device drivers: If the login session is not the root user then ALSA, USB, my wireless card, and the battery monitors (portable) aren't working.
Right now, almost everything in /usr/share, /usr/lib, and /usr/src is rwxr-xr-x root root (trying to get it to work again), but as that didn't fix the drivers or whatever I now am going through and examining the permissions from the live installer copy.

Anyway, so I will continue doing this as an educational exercise for myself. What I am curious to know is whether it is possible that the problem has spread from /usr, perhaps causing some changes in /etc during boot when it found undesirable permissions modes.
Er, I'm not completely new to Linux but I have some gaps in my knowledge, such as I yet don't even know where I can view my boot log — though I maybe will have found that information by the time you read this.

There's little that I can post online, because it would probably be a huge ls -lAR dump.

Thank you for taking the interest in helping, et c.

Have a gander at /var/log/boot.log