I thought I had lost this when I started working in Unity3d, and I never needed more than now. Although I can reference the code from other project easily enough using MonoDevelop, the code is only half the story. Often, the scene can tell you more than the actual code can. And although I've been programming in it for a while now, I still find the plethora of Unity tutorial online to be invaluable in my endeavour to better myself.
It turns out it's absolutely possible to do this. Just not as straight forward as you would launch it normally. There's a nice command line page for unity here, the option that is important for us right now is the -projectPath.
Now from the terminal, I type:
/Applications/Unity/Unity.app/Contents/MacOS/Unity -projectPath /Users/shadhex/Documents/Unity/
The .command extension insures that I can launch the script from Finder. The & lets the application launch in the background so that I can close my terminal window without shutting down Unity.
Hopefully that helps all the learn by example type coders out there!