This is a quick and dirty port of the completions generated by RubyMotion.tmBundle. As such, I'm not sure if there are any missing methods or not, but feel free to contribute if you find some.
Put this package into your Sublime Text 2 packages folder:
Windows: %APPDATA%\Sublime Text 2\Packages
OS X: ~/Library/Application Support/Sublime Text 2/Packages
Linux: ~/.Sublime Text 2/Packages
Because iOS 6.0 is currently under a NDA you will have to generate the completions manually. To do this you will first need to update RubyMotion to support iOS 6.0 using the following commands in your OS X Terminal:
cd /Library/RubyMotion/data/6.0; rake update
Now you can run the following command to regenerate the completions:
ruby ~/Library/Application\ Support/Sublime\ Text\ 2/Packages/RubyMotion/Support/compile_completions.rb > ~/Library/Application\ Support/Sublime\ Text\ 2/Packages/RubyMotion/RubyMotion.sublime-completions
Please note that I don't currently have access to the iOS 6 beta so if you run across any problems with the generations, please let me know
Inside your RubyMotion project just start typing the name of a method and the autocomplete window will pop up.
Press enter/return to trigger the completion.