You will need:
Repair Putty aka Mighty putty - I got mine at Dollarama for 2.00
Small Phillips Screwdriver - not sure what size...
Black paint - latex, whatever
Unfortunately I didn't get the idea to make this repair an Instructable until half way through the process so you'll have to make do with the pics I took.
Step 1: Isolating the broken piece.
Next you need to detach the actual cup from the broken piece (looks like an inverted Y).
See the pictures for explanation.
Step 2: Attaching with putting
When doing this part, make sure you avoid getting putty in the area where the plastic has to snap back in and be screwed on.
You need to work quick as the stuff starts to harden about 3-5 minutes.
Step 3: Finishing touches
Once you've done that, you can add some paint to make it blend in.
All that remains is to snap the piece back on the ear cup and then reattached the whole thing to the headphones... Voila good as new.
Check out the power of mighty putty here.