Use iChat behind a company firewall: "Many companies block the default port used by most IM clients. However, you can change iChat's port from 5190 to 80, which is the same port your browser uses. Open iChat, then go to Preferences » Accounts » Server Settings, and change the port number from 5190 to 80. You must be offline (iChat: Log out of AIM) before you can make this change.
This will allow iChat to connect to the IM servers, and off to text messaging you go. I'm pretty sure this tip will only work for text messaging, since voice and video chat use ports that are not changable in iChat.
[robg adds: Any impacts on your employment status as a result of using this tip are solely your responsibility :).]"