Probably this is something that every Mac owner learns in kindergarten, but just in case someone else got through school without it, here it is:
I had a Toast image (with the file extension .toast) that 10.3 Disk Utility would not burn to a blank CD -- the filename was grayed out in the list of files available for burning.
I simply added the extension .iso after the extension .toast, and Disk Utility burned it with no problems.
[robg adds: This older hint touched on a related topic, but it seems behaviors may have changed with Disk Copy in 10.3...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040701101907400