What exactly gets wiped when unlocking Galaxy Nexus?
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.