If you've got an Intel Mac, the process for getting it to boot from an external FireWire drive is different than it is for a PowerPC Mac -- an Intel boot drive requires a different partition scheme than does a PowerPC boot drive.
Jonathan Rentzsch has written a very detailed article on these differences for TidBITS, explaining just what is required to make it work, and some pitfalls to be aware of going forward.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060202061257652