As posted here earlier, it is possible to create a "Universal OS X" installation disk.
But what's even better is one can create an Universal NetBoot Image, too! Simply image the partition, open the .plist file in the image folder, and add the entry ppc to architectures. Now you can activate the image, and even select the supported platform - PowerPC, Intel or both: Universal.
I tested this with an iMac G5 17" and an Intel iMac 17" (the Intel iMac is much faster).
[robg adds: I wish I had more detail to add on this (such as the exact name of "the .plist file"), but I can't test it myself, and the author left no contact info.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060323085045578