From the 'Counting' paper:

] A keyed generator, as is used in OpenBSD and FreeBSD,
] provides some protection, but one needs to be careful
] to avoid duplication if the generator is rekeyed
] periodically.

This feature can be enabled in FreeBSD with a kernel compile option.
From the /usr/src/sys/i386/conf/LINT file:

]] # RANDOM_IP_ID causes the ID field in IP packets to be randomized
]] # instead of incremented by 1 with each packet generated. This
]] # option closes a minor information leak which allows remote
]] # observers to determine the rate of packet generation on the
]] # machine by watching the counter.
]] options RANDOM_IP_ID

RE: A Technique for Counting NATted Hosts [PDF]