A standing opinion about Evernote is that it's fantastic at accepting information but terrible at giving it up again. You can search from within the app, but any kind of integration with other apps is only barely possible—by, say, linking to the Evernote web client.

OmniFocus, for its part, is exceptional at helping you organize your life, but really isn't designed to be a repository of detailed information, so integration with Evernote is a natural desire. For example, I have an OmniFocus single-action list which I use to store blog post ideas; it's a natural next step to have an Evernote note associated with each containing notes and drafts.

So here's the method (admittedly, somewhat manual) that I've been using to keep references to Evernote notes with associated OmniFocus actions.

1. Create the note. This is the part that really ought to be automated, but isn't (some AppleScript-fu, which I unfortunately do not possess, could probably eliminate or ease this step. Suggestions welcome). Go into Evernote and make a new note, presumably titled similarly to your OmniFocus action.
2. Pull up the note in Finder. Herein lies the only interesting part. Since Evernote notes show up in Spotlight searches, there is at least some avenue for getting a reference to the note outside of Evernote itself; we will exploit this.
   
   
   • Open up Finder and start a new search (type a character in the search box, then delete it).
     
     
     
   • Click the + button to add a new criterion, where Kind is Other. Type "Evernote Note" into the box. You can restrict this search to your home folder if you have other Evernote users on the same computer. Sort descending by Last Opened.
     
     
     
   • Click "Save." Save it as "Evernote" and add it to your sidebar.
     
     
     
   
   Now selecting this saved search in Finder will pull up all your notes in an order conducive to this process. If you just created the note, it'll be right on top; otherwise, you can progressively filter the search by some text.
3. Drag the note to the OmniFocus. Just drop it on the action to get a link to Evernote in the notes field.
   
   
   

Nothing fancy, nothing new, and as I said, still more manual than one would hope. A button in OF that automatically creates a note in Evernote and plugs the reference into the action would be sweet. Still, until somebody writes that or I manage to find the time to learn some AppleScript, this method seems to be working pretty well for me.