How are transaction hashes calculated?
Taking the sha256 hash of that hash gives
  
  3b a3 ed fd 7a 7b ..............
  
  But the real transaction hash according to blockexplorer.com is
  
  .............. 7b 7a fd ed a3 3b


The answer I was getting was correct, but bytewise reversed.  I need to get used to Bitcoin using little-endian storage.