I've looked and looked for a Sublime action for Finder but nothing seemed to work as I'd prefer so I decided to roll my own. Sublime It! is a very simple Automator app that makes use of the subl executable (bundled with Sublime) to launch the current Finder selection in Sublime.
It works as you'd expect for multiple selections, files & folders, white space in paths, etc. When launched, it will open highlighted files in the topmost window of Sublime (or launch a new one if Sublime isn't running). As a bonus, you can hold down shift while clicking the action, and this will cause the current selection to open in a brand new Sublime window. This feature makes use of the excellent checkModifierKeys binary by Stefan Klieme (StefanK).
Download, unzip and copy Sublime It!.app to your applications folder. ⌘⌥+drag the app to your finder toolbar.
NB First time you run the action, it may not work due to the usual OSX security dialog about the App being downloaded and whether you want to run it. Just hit OK and try again - it will work from now on :)