i'm also trying to do this, using an AVRISPmkII to update bootloader on an Arduino NG with atmega8 to eliminate startup delay. can you detail exactly what changes you made to makefile, and what command line instruction you used? (i assume you used avrdude.)
i would like to use this improved ADABOOT
http://www.1strecon.org/TheShoppe/freeduino/ADABOOT.shtml
rather than the original
http://www.ladyada.net/library/arduino/bootloader.html
but it looks like it will be more work making the newer one work with atmega8...