CrossRoads:
Read this thread
http://arduino.cc/forum/index.php/topic,59701.0.html
No problem, I will alter the instructions to give two entries, one for 16Mhz and one for 20Mhz. If someone has a good stable 16Mhz hex for the 1284p I can include that otherwise will need to work out how to create them.
Giving both speeds sounds ideal. Plenty of applications would benefit from the increased speed but not be caused an issue by the listed problems. Plenty of applications simply need the extra io ports which was my initial reason for going down the 1284p route.