Some offices and schools block iChat. One possible way to fix that problem is by enabling SSL connections in iChat. Go to Prefrences » Accounts » Server Settings, and check the Use SSL box.
Note: You must be offline in order to change this setting.
[robg adds: I don't have any way to test whether this works or not.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080128103351656