Seeing this here as well, here's some background
mkent@el-test:~/git/chef-repo (master)$ knife cookbook upload teamspeak
INFO: ** teamspeak **
INFO: Copying from /home/mkent/git/chef-repo/cookbooks/teamspeak to /tmp/chef-teamspeak-build.24527.0
INFO: Nothing to copy from /home/mkent/git/chef-repo/site-cookbooks/teamspeak
INFO: Testing /tmp/chef-teamspeak-build.24527.0/teamspeak/metadata.rb for syntax errors...
INFO: Testing /tmp/chef-teamspeak-build.24527.0/teamspeak/recipes/default.rb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/teamspeak.conf.erb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/server.ini.erb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/port_teamspeak.erb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/demo.php.erb for syntax errors...
INFO: Generating metadata for teamspeak
INFO: Generating from /tmp/chef-teamspeak-build.24527.0/teamspeak/metadata.rb
INFO: Generated /tmp/chef-teamspeak-build.24527.0/teamspeak/metadata.json
INFO: Creating tarball at /tmp/chef-teamspeak.24527.0.tar.gz
INFO: Upload complete!
mkent@el-test:~/git/chef-repo (master)$ echo $?
0
Hooray!
Oh wait
merb : worker (port 4000) ~ Started request handling: Fri Apr 09 14:28:00 -0700 2010
merb : worker (port 4000) ~ Params: {"name"=>"teamspeak", "format"=>nil, "action"=>"create", "controller"=>"chef_server_api/cookbooks", "file"=>{"content_type"=>"application/octet-stream", "size"=>3748, "tempfile"=>#<File:/tmp/Merb.24523.0>, "filename"=>"chef-teamspeak.24527.0.tar.gz"}}
merb : worker (port 4000) ~ Permission denied - /xvar - (Errno::EACCES)
/usr/lib/ruby/1.8/fileutils.rb:243:in `mkdir'
/usr/lib/ruby/1.8/fileutils.rb:243:in `fu_mkdir'
/usr/lib/ruby/1.8/fileutils.rb:217:in `mkdir_p'
/usr/lib/ruby/1.8/fileutils.rb:215:in `reverse_each'
/usr/lib/ruby/1.8/fileutils.rb:215:in `mkdir_p'
/usr/lib/ruby/1.8/fileutils.rb:201:in `each'
/usr/lib/ruby/1.8/fileutils.rb:201:in `mkdir_p'
/usr/lib/ruby/gems/1.8/gems/chef-server-api-0.8.10/app/helpers/tarball_helper.rb:70:in `expand_tarball_and_put_in_repository'
/usr/lib/ruby/gems/1.8/gems/chef-server-api-0.8.10/app/controllers/cookbooks.rb:168:in `create'
/usr/lib/ruby/gems/1.8/gems/merb-core-1.0.15/lib/merb-core/controller/abstract_controller.rb:315:in `send'

Seeing this here as well, here's some background
mkent@el-test:~/git/chef-repo (master)$ knife cookbook upload teamspeak
INFO: ** teamspeak **
INFO: Copying from /home/mkent/git/chef-repo/cookbooks/teamspeak to /tmp/chef-teamspeak-build.24527.0
INFO: Nothing to copy from /home/mkent/git/chef-repo/site-cookbooks/teamspeak
INFO: Testing /tmp/chef-teamspeak-build.24527.0/teamspeak/metadata.rb for syntax errors...
INFO: Testing /tmp/chef-teamspeak-build.24527.0/teamspeak/recipes/default.rb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/teamspeak.conf.erb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/server.ini.erb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/port_teamspeak.erb for syntax errors...
INFO: Testing template /tmp/chef-teamspeak-build.24527.0/teamspeak/templates/default/demo.php.erb for syntax errors...
INFO: Generating metadata for teamspeak
INFO: Generating from /tmp/chef-teamspeak-build.24527.0/teamspeak/metadata.rb
INFO: Generated /tmp/chef-teamspeak-build.24527.0/teamspeak/metadata.json
INFO: Creating tarball at /tmp/chef-teamspeak.24527.0.tar.gz
INFO: Upload complete!
mkent@el-test:~/git/chef-repo (master)$ echo $?
0
Hooray!
Oh wait
merb : worker (port 4000) ~ Started request handling: Fri Apr 09 14:28:00 -0700 2010
merb : worker (port 4000) ~ Params: {"name"=>"teamspeak", "format"=>nil, "action"=>"create", "controller"=>"chef_server_api/cookbooks", "file"=>{"content_type"=>"application/octet-stream", "size"=>3748, "tempfile"=>#<File:/tmp/Merb.24523.0>, "filename"=>"chef-teamspeak.24527.0.tar.gz"}}
merb : worker (port 4000) ~ Permission denied - /xvar - (Errno::EACCES)
/usr/lib/ruby/1.8/fileutils.rb:243:in `mkdir'
/usr/lib/ruby/1.8/fileutils.rb:243:in `fu_mkdir'
/usr/lib/ruby/1.8/fileutils.rb:217:in `mkdir_p'
/usr/lib/ruby/1.8/fileutils.rb:215:in `reverse_each'
/usr/lib/ruby/1.8/fileutils.rb:215:in `mkdir_p'
/usr/lib/ruby/1.8/fileutils.rb:201:in `each'
/usr/lib/ruby/1.8/fileutils.rb:201:in `mkdir_p'
/usr/lib/ruby/gems/1.8/gems/chef-server-api-0.8.10/app/helpers/tarball_helper.rb:70:in `expand_tarball_and_put_in_repository'
/usr/lib/ruby/gems/1.8/gems/chef-server-api-0.8.10/app/controllers/cookbooks.rb:168:in `create'
/usr/lib/ruby/gems/1.8/gems/merb-core-1.0.15/lib/merb-core/controller/abstract_controller.rb:315:in `send'