This has been bugging me for a long time, and I don't have a Google calendar to use as a KLUDGE like others have recommended - plus, it just feels wrong. But today I can up with a quick fix that I hope will work going forward.
I simply downloaded the ics file from the TripIt link, which looks something like this (yes, I've changed the GUID):
https://www.tripit.com/feed/ical/private/0C6C4AA6-39A500CF87B79E7AEFAF250F75A77E63/tripit.ics
Then I opened the ics file from my Downloads folder. I was prompted to answer several questions, but it worked, and the calendar was added.
Now the question is whether it will sync over time.