Just wasted an hour fighting a bug in Rubymine 1.1.1 that wouldn’t let me open files. Every once in awhile, it just decides that it doesn’t want to open files of a certain type (in my case, all .html.erb and .rhtml files), regardless of how you try to open them (click in file browser, choose File -> Open, etc).
And now, ladies and gentlemen, without further ado, the fix:
File -> Invalidate caches
Close and re-open Rubymine