MacBook Air mid 2011 - Handoff not supported
As of 10/23/2014 The creator of the tutorial below (@dokterdok) has released a software tool to automate the process.

Below is from the original macrumors post...


  (@dokterdok)
  
  Update 23.10.2014: I've released a Continuity Activation Tool. No more
  painful manual hacking. Link.







This is how i fixed it (according to this wonderful MacRumors Tutorial mentioned by @Hank):


  
  Download and install 0xED in your Applications folder, you can get it from here: http://www.suavetech.com/0xed/
  Download and install KextDrop in your Applications folder, you can get it from here: http://www.cindori.org/software/kextdrop/
  Open Terminal and type : sudo nvram boot-args="kext-dev-mode=1"
  Restart your machine.
  Open Finder, type CMD+Shift+G and go to /System/Library/Extensions/
  Copy those two files to your desktop and then delete the originals: IO80211Family.kext IOBluetoothFamily.kext
  Go to Terminal, and type: ioreg -l | grep "board-id" | awk -F\" '{print $4}'
  Your result should look like : Mac-742912EFDBEE19B3, (yours might be different.). Copy it to your clipboard (Select and CMD+C). Don't
  close terminal yet.
  Open Finder and hit CMD+Shift+G, go to ~/Desktop/IO80211Family.kext/Contents/Plugins/AirPortBrcm4360.kext/Contents/MacOS
  Note: if you have a different OS language, you might need to replace
  "Desktop" in the path with your local translation.
  Right Click on AirPortBrcm4360, and choose OPEN WITH: OTHER: 0XED
  Press CMD+F, search for "Mac-" (without the quotes). Make sure the search type is Text and not HEX.
  Look at the first result, for example, it might be Mac-C3EC7CD22292981F
  Go back to the FIND box, in the FIND field enter that first "Mac-" number that you found.
  In the REPLACE field, enter the number you got in Terminal, (remember you left it open.)
  Click REPLACE
  Repeat steps 11 to 15, but this time for the last "Mac-" result that you can find in the file.
  Click SAVE
  Open Finder and hit CMD+Shift+G to open the "Go to" popup, and go to ~/Desktop/IOBluetoothFamily.kext/Contents/MacOS
  Right Click on IOBluetoothFamily, and choose OPEN WITH: OTHER: 0XED.
  Press CMD+F.  In the SEARCH field type: MacBookAir Click FIND NEXT.
  You will find a block of text containing 'MacBookAir4,1''MacBookAir4,2''Macmini5,1''Macmini5,2''Macmini5,3'
  Manually replace the numbers with ones only so they all look like:
  'MacBookAir1,1''MacBookAir1,1''Macmini1,1''Macmini1,1''Macmini1,1'
  Click SAVE (the red circle button) and quit 0xED.
  Open KextDrop, and Drag and drop the IO80211Family.kext that you modified into the window. 
  Click INSTALL. 
  Repeat steps 23 and 24 but for your modified IOBluetoothFamily.kext
  Go Back to Terminal, and type the following commands. Wait for each one to finish. sudo kextcache -system-prelinked-kernel and
  sudo kextcache -system-caches
  Reboot. Make sure you have an Internet connection for the next steps.
  
  
  Last: SignOut and in again your Mac and iPhone, reboot both.




Here is how it should look like after this: