Okay, I finally decided to log out of my X11 session on my netbook (Fedora) and found that my memory was wrong.
last does not report logins to X11 desktop sessions.
Is there a command that does? I've been looking around the web, but so far google just tells me that X11 has an error log, which I know about.
auth.log has the information, but it's mixed with a bunch of other stuff.
lightdm/lightdm.log seems to have the information, too, but the time shown looks to be time from boot. I could write a script to extract just the login lines from auth.log, but I want to think someone has already done it. I mean, sure, grep sort of does the job:
sudo grep "lightdm: pam_unix(lightdm:session): session opened for user" /var/log/auth.log