Currently, you fail:

[adam@latte](127)(master*)% knife cookbook delete opsmaster ~/src/sandbox/opscode/opsmaster
zsh: correct 'cookbook' to 'cookbooks' [nyae]? n
Do you really want to delete opsmaster ? (Y/N) y
WARN: HTTP Request Returned 404 Not Found: No routes match the request: /organizations/opsmaster/cookbooks/opsmaster/
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/net/http.rb:2097:in `error!': 404 "Not Found" (Net::HTTPServerException)
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/rest.rb:216:in `api_request'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/rest.rb:267:in `retriable_rest_request'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/rest.rb:197:in `api_request'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/rest.rb:106:in `delete_rest'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/knife/cookbook_delete.rb:29:in `run'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/knife.rb:309:in `call'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/knife.rb:309:in `delete_object'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/knife/cookbook_delete.rb:28:in `run'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/lib/chef/application/knife.rb:115:in `run'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.0.a8/bin/knife:25
from /usr/local/bin/knife:19:in `load'
from /usr/local/bin/knife:19

Would be better to pick a version and delete that.