Normally, when iChat launches, it performs a bandwidth check against bwdstart://snatmap.mac.com:16384. This can be verified by launching iChat with debugging enabled, as found elsewhere on the net. Launch iChat in Terminal with this command:
/Applications/iChat.app/Contents/MacOS/iChat -errorLogLevel 7
This results in a complete lack of consistency with regard to the amount of bandwidth allocated to iChat. For example, very little bandwidth will be allocated to iChat if the internet "pipe" is full when iChat is launched. Conversely, if there is little to no internet traffic when iChat is launched, a large amount of bandwidth will be allocated.
Adding a firewall rule that blocks iChat's attempt to contact this "bandwidth server" forces a fallback to the QuickTime Streaming Speed setting. This will allow you to have a modicum of control over the amount of bandwidth that iChat uses. Here is the command you will need to enter in Terminal (without line breaks) to enable this rule:
sudo ipfw add deny log logamount 65535 ip from any to 17.250.248.149 out via en0
Of course, if you are using something other than built-in ethernet, you will need to substitute the proper interface name (such as en1 for an Airport connection). These can be viewed by issuing an ifconfig -a in Terminal. You will also need to delete iChat's preference files (~/Library -> Preferences -> com.apple.ichat.*) in order for this to take effect. Hopefully someone will find this to be useful!

