I switched to the Guest profile from my profile, used Guest for a couple of hours, then clicked "Log Out..." from the power/session menu. Since logging out from Guest, pantheon-greeter has been stuck in a crash loop. At times, I can see my profile selected, but the wallpaper from Guest is displayed, and the greeter crashes before I am allowed to type in my password. After the crash, pantheon-greeter seems to restart, leading to another crash, rinse and repeat.
Starting elementary in recovery mode allows me to start pantheon-greeter with no problems (using service lightdm start), but as soon as I reboot, the crash loop starts again. Currently, I have configured my profile to auto-login, so I can actually use my computer. I have applied all available updates, and I completely removed pantheon-greeter with Synaptic, then reinstalled, only to find the same crash loop exists.
Okay, I think I've narrowed down what was causing the crash loop, and it has nothing to do with the guest profile.
The same day that I used the guest profile, I also changed my background picture to something I downloaded. The file was located in ~/Pictures, and it turns out that THIS was the cause of the crash. Changing my desktop background to one of the pictures included with elementary solved the problem.
So, to sum up, anything other than the default backgrounds makes (my) pantheon-greeter crash over and over again.