Steps to reproduce (much easier in 1.7snapshots)

• Type the following:

  /summon Enderman <x> <y> <z> {carried:{id:<blockID>}}

• Make sure the block ID is anything other than TNT, grass/dirt, sand, gravel, vines, flowers, pumpkins (see list of things endermen can hold on wiki)
• It will spawn not carrying any block as the command would have failed, this also limits what you can get endermen to hold