Just finished up a couple of Arduino ISP sheilds one for the Atmel ATmega 328's and his little brother the ATtiny 45/85.
The ATtiny 45/85 shield is based on the schematic here. With the addition of a 10uf capacitor between GND and Reset which is selectable through a jumper .
The cap is detailed as being necessary for the UNO but i found i needed it even with my Duemilanove to program the chip reliably .
As for his big brother he is based on the Mega ISP schematic here .
Don't let the name fool you its not aimed at the Arduino Mega more to do with the range of chips that it programs which happen to be called "ATmega".
The Code for the Mega ISP Shield comes with the Arduino IDE and should come in handy for programming the bootloaders on blank 328's.
I will have to report back and share the Eagle files if all turns out well as i dont have any ATmega 328 chips floating around to test it .