Do any modern airliners have a non-flat floor in the passenger cabin?
Some Airbus aircraft have this characteristic, in the rear section of the cabin floor. You can see this in the following image. The rear passenger windows slope up following the floor slope.



A340 line drawings; image from wikimedia.org

This image of the upper cargo deck of A330 MRTT upper cargo deck (which would be the cabin floor in the passenger version) shows the upward slope.



A330MRTT cabin floor; image from thinkdefence.co.uk 

Apparently, this is a common trait in A300/310/340/330's to allow more containerized cargo in the rear.