According to the Wikipedia IPv6 article, Privacy extensions are, except for the Windows platform and Mac OS X since 10.7 as well as iOS since version 4.3, not enabled by default.
In theory, one can enable the IPv6 Privacy Extensions on all interfaces at once using sysctl like this:
sudo sysctl net.ipv6.conf.all.use_tempaddr=2
However, this currently doesn't work as expected, so I'm using this one-liner in /etc/rc.local:
for IF in `/bin/ls /proc/sys/net/ipv6/conf/*/use_tempaddr` ; do echo 2 > $IF ; done
This also sets "use_tempaddr" for "default", which means it should also apply to interfaces added to the system afterwards.
A simple check to verify that the new configuration is working: ipv6-test.com will print your MAC address when available...