Searching the web for a solution to my "Invalid node structure" problem seems to point me to using TechTool Pro - a $99 3rd party utility. DiskWarrior seems to work as well, but only for non-Intel Macs. TechTool Pro is currently the only app available that supports Intel Macs.
I still intend to check Apple's support information to see if they have a solution for the problem that does not require a 3rd party application. Disk Utility does not work, even if I boot from an external device.
What does invalid node structure mean anyway? What causes it? Is it hardware related (i.e., is my HD failing)?