We have a sinatra app nested inside of a rails 3 app, and the files don't reload like rails models do. Of course, they don't get reloaded in dev like the rest of rails does, so this may be a separate issue...
I have a Sinatra app mounted in my Rails routes. I have reloading working with Spork for testing. Here's how:
mount SinatraApp::App => "/sinatra"
class SinatraApp < Sinatra::Base