What exactly gets wiped when unlocking Galaxy Nexus? I've seen a few guides on how to unlock the Galaxy Nexus to prepare it for rooting, and they say that the fastboot oem unlock command will "wipe your personal data".

I'm a little unclear on what gets wiped during this step. Is it just the /sdcard area? is it everything but /system? Something in between?

I know that adb backup can (at least theoretically) back up everything except /system while unrooted (see a recent question of mine). But I wonder if I would need to do an adb restore of everything after the unlock step in order to keep all of my data and settings.
Both /data and /sdcard are erased when you do fastboot oem unlock.  So you lose not just files on the "SD card", but all installed applications and their internal settings and data.

Yes, you can use adb backup to backup your data and then restore it later, either with adb restore or Titanium Backup Pro.  However, when I did this (using adb restore), my MMS messages were lost; it seems that the backup didn't save them, though maybe I just did something wrong.

There's also a way to root without unlocking your bootloader.  Once rooted, you can use BootUnlocker to unlock it without wiping data.