Is it safe to delete the /Developer folder on Mac OS X 10.7.5 (Lion)?
Yes, it looks like it should be safe to remove /Developer if you have already installed version 4.3 or later of Xcode. The Xcode section of the Apple Developer Support Center actually tells you to do this (emphasis mine).


  I've installed the latest version of Xcode from the Mac App Store, but a previous version of Xcode (4.2.1 or earlier) keeps launching. What should I do?
  
  
  
  To run the latest version of Xcode, make sure you are launching Xcode.app from the Applications folder. Xcode 4.3 and later is packaged on the Mac App Store as a single application and installs in your Applications folder as Xcode.app.
  
  Previous versions of Xcode placed the tools and frameworks in the Developer folder. You can delete the older version of Xcode by dragging the Developer folder to the trash.


Furthermore, as pointed out by bispymusic, /Developer no longer exists with Xcode 4.3. Specifically (emphasis mine):


  The simplification afforded by repackaging Xcode 4.3 as a single app bundle eliminates the need for the /Developer directory containing prior versions of Xcode. As a result, the Install Xcode application and the uninstall-devtools command line script are also no longer needed.
  
  Note: The first time you run Xcode 4.3, you are prompted with a dialog to delete the Install Xcode application from the Applications directory if one is resident, and to remove any older installation of Xcode (Figure 4). These actions are optional, Xcode 4.3 can co-exist with older installations of Xcode on the same Mac. Some operations may require an administrator password to enable them if you switch from one version of Xcode to another.
  
  Figure 4  Option to remove older versions of Xcode
  
  
  
  If you choose not to delete these items on the first launch of Xcode 4.3, you can remove the /Developer folder and the Install Xcode application by dragging them to the Trash at any time.