What is the optimum distance between a micro-controller and a crystal on a PCB?
Once again, Atmel app notes to the rescue:

Atmel AVR 8-bit Microcontrollers Application Note - AVR042: AVR Hardware Design Considerations

Crystal/oscillator advice starts on page 13 and on page 16, it recommends a "Short distance between the crystal/capacitors and the MCU".  Also, an example layout on page 18 shows how they laid-out the crystal circuit.