Is it still possible to assign combat damage, and return the creature back to its owner's hand?
Yes, this is exactly one of the tricks which is no longer possible after the Magic 2010 rules changes (scroll down to 5) Combat Damage No Longer Uses the Stack). It's even mentioned explicitly:


  How can you Unsummon your creature and have it still deal damage?


This didn't really make sense from a flavor perspective, and therefore they've fixed it:


  The Fix: As soon as damage is assigned in the combat damage step, it is dealt. There is no time to cast spells and activate abilities in between; the last time to do so prior to damage being dealt is during the declare blockers step.


So you either have to Unsummon your creature before damage is assigned (but then the other creature survives as well) or both of them will die.