I usually like to be prepared for my on-site interviews by reading Glassdoor beforehand, but it turns out I should have looked carefully here even before the phone screen (as I got blasted out of the water by relatively elementary questions). Hopefully you can avoid the embarrassment that I had and be somewhat better prepared.
So TripAdvisor is looking for mobile engineers, but the immediate takeaway I got from Dan the hiring manager is that if you have any specialty (e.g. Android, iOS, RoR, backend, etc.), you'll be expected to be able to work in everything else as well and not necessarily just your specialty. This may be good for folks looking for broad experience, but not so good for a developer looking to sharpen or go very deep on one particular platform.
They also talked about one week sprints, which seems like a lot of pressure to do potentially larger tasks. TripAdvisor also doesn't have architects, which kind of explains why their current mobile offerings are a bit of a user-unfriendly, haphazard mess.
About this point I started getting peppered with general computer science questions, which weren't the platform specific questions I had been expecting. Expect to get grilled on: Big-O/Little-O complexity, hash tables, trees and sorting algorithms; all things that grad students think about on a regular basis but not necessarily things application engineers who are more focused on getting apps implemented.
As much as I like the concept of TripAdvisor and the (equally clunky) website, I was pretty well shaken up by the experience of talking with the hiring manager enough to know that this wouldn't be a place where a mobile engineer could be inspired to focus and do his/her best work.