In the Atmel's 'AVR Hardware Design Considerations' [ link ] document, page 6, item 4.1.1, it states:
Shared use of SPI programming lines If additional devices are connected to the ISP lines, the programmer must be protected from any device, other than the AVR, that may try to drive the lines. This is especially important with the SPI bus, as it is similar to the ISP interface. Applying series resistors on the SPI lines, as depicted in Figure 4-2, is the easiest way to achieve this.
However, there is no mention anywhere of what size those resistors should be. Anyone want to take a gander at this?