Everyone knows about Data Detectors in Mail, which automatically detect info such as dates, times, names, and addresses, and let you use that data directly from the message in iCal. You can enable Data Detectors in iChat messages by entering the following into the Terminal (when iChat's not running):

defaults write com.apple.iChat EnableDataDetectors 1

To disable this, just reenter the code into Terminal, changing the 1 to a 0.

[robg adds: In testing this, it seems the behavior of Data Detectors in iChat is somewhat flakey (which may be why they're disabled by default). When activated, they seem to work at first, but then just stop working after some random amount of time. You can make them active again by simply selecting a different view, then reselecting your preferred view (via the View -> Messages -> Show as... menu). They will, however, then stop working again at some point, requiring the same solution.]