After some more investigation on this issue, these two messages caught my attention:
http://lists.apple.com/archives/usb/2010/Nov/msg00040.html
http://lists.apple.com/archives/usb/2010/Nov/msg00041.html
Given that:
A) the USB probe output reported in the first message is related to a full speed USB device implementation;
B) at the date of the second message the latest released version of Mac OS X was 10.6.4; version 10.6.5 was released 5 days later (Mac OS X Snow Leopard - Wikipedia);
C) during the test phase of the Due no issue was experienced when the Due was still configured as a full speed device (I'll check this again in the next days, just to be shure...). Only when the high speed configuration is enabled the serial port is not detected (as cmaglie already pointed out);
this is my educated guess: is it possible that something has been fixed in 10.6.8?
In the second message Paul Stoffregen says that (only) mouse and disk appear on OS X (he doesn't mention the CDC-ACM Serial that is detected on Windows and Linux), but we experienced a different behaviour (as described in C) ).
Provided that the device configuration of Paul Stoffregen's demo is different (Miscellaneous/Common Class device), is it possible that a similar configuration can solve the problem? Of course this should be considered only a workaround, but, in my opinion, a better one compared to the 1) mentioned in my previous post.