Q: Can [an almighty being] create a rock so heavy that he himself cannot 
   lift it?

A: Imagine one can take all the mass in the Universe and clot it together. 
   There is now one lump of mass with a single center point of gravity 
   (probably a singularity/black hole), floating in the vacuum that is 
   Space.

   So far, so good. What did you mean again by "lift"? Simply *moving*
   all this mass takes a near-zero amount of energy. After all, each 
   action causes an equal and opposite reaction.
   So the smallest butterfly-sneeze would set the lump of mass in motion
   (relative to what?) albeit ever so slowly. 

   Problem is, with all mass packed into a single chunk,
   there won't be a source for this butterfly-sneeze. 

   And without a frame of reference, there is no telling how fast this
   chunk of mass moves *regardless* of its speed.

   Lifting something implies having an object and moving it away from
   a larger objects' center of gravity, while being supported by that 
   larger object. As there is no "larger" object to move "all the mass 
   in the universe" away from anymore, it sure is possible for an 
   almighty being to create a rock so heavy that it *can* no longer be 
   lifted due to the fact that the definition of "lifting" can no longer
   be applied. Mind you that this is not due to a lack of "almightiness"
   but due to a lack of insight in physics by the person asking the 
   question.

   By the way. 

   Note that this eventuality (all matter in the universe clumping 
   together) has already taken place, no Almighty Being required.