Yeah, this uncanny similarity is more disconcerting than the "lady parts" people see.

There needs to be an easy way to search for existing logos by image similarity... the USPTO has an extensive categorisation method for trademarks ( http://tess2.uspto.gov/tmdb/dscm/index.htm ) but what's really needed is something more visual. Using Google on a greyscale version of Airbnb's logo didn't yield anything geometrically similar.

There is; it's called a trademark search, and it's a matter of public record: "The trademark examining attorney has searched the Office’s database of registered and pending marks and has found no conflicting marks that would bar registration..." (http://tsdr.uspto.gov/documentviewer?caseId=sn86160387&docId...) ~The USPTO's response to this Jan 8, 2014 filing: http://tsdr.uspto.gov/documentviewer?caseId=sn86160387&docId...

Looks like a possible add-on feature for http://www.brandsoftheworld.com/