commands im using

cp /usr/local/Cellar/mongodb/1.6.5-x86_64/org.mongodb.mongod.plist ~/Library/LaunchAgents
launchctl load -w ~/Library/LaunchAgents/org.mongodb.mongod.plist

Then reboot login and

ps -e | grep mongo

nothing

org.mongodb.mongod.plist file

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>org.mongodb.mongod</string>
  <key>ProgramArguments</key>
  <array>
    <string>/usr/local/Cellar/mongodb/1.6.5-x86_64/bin/mongod</string>
    <string>run</string>
    <string>--config</string>
    <string>/usr/local/Cellar/mongodb/1.6.5-x86_64/mongod.conf</string>
  </array>
  <key>RunAtLoad</key>
  <true/>
  <key>KeepAlive</key>
  <true/>
  <key>UserName</key>
  <string>pat</string>
  <key>WorkingDirectory</key>
  <string>/usr/local</string>
  <key>StandardErrorPath</key>
  <string>/usr/local/var/log/mongodb/output.log</string>
  <key>StandardOutPath</key>
  <string>/usr/local/var/log/mongodb/output.log</string>
</dict>
</plist>