The knife-ec2 plugin does not explicity depend on 'chef'. The historical reason is unknown, adding it back.
merged to master
I think our concern was that if you had a version of chef installed that was not the latest version and you installed the latest version of knife-ec2 using RubyGems, it would pull in the latest version if chef as well. I verified this doesn't happen with RubyGems 1.3.7 and 1.8.2 installing from a Gem, but I did not test it installing from a Gem server.