Not an issue for me. I have a Ruby on Rails project synced in my DropBox folder (Well, nested, so: ~/Dropbox/Dev/Proj/...) and I have the whole thing opened as a project. It's actually a pretty nice set-up and I'm really loving Sublime for it. I'm using dev build 2135 on OS X Lion 10.7.2.
Edit: Since it's just Ruby files it sounds like it might be some sort of theme issue, maybe? I'm using the Soda Dark theme with Espresso Libre color scheme.