IPv6 auto configuration on Linux will normally assign the same address every time when connecting to a specific network. This address would normally be formed from the network prefix and local interface MAC address.

When using IPv4 your identity is slightly masqued by NAT. Although your single public IP address may be tracked, it is hard to track individual devices that exist on the local network. This is not the case with IPv6 where there is no NAT. Every device has a visible unique public IP address that rarely changes.

RFC4941 (obsoletes 3041) defines privacy extensions to IPv6 which will randomly assign an additional Global IPv6 address to the interface. This additional random address will have the same network prefix and be used for outgoing internet connections.

To enable the privacy extensions under Linux (using Debian Wheezy):
echo 2 > /proc/sys/net/ipv6/conf/${ifname}/use_tempaddr
replace ${ifname} with the name of your interface, eg wlan0

Possible values for this setting:
(0=off, 1=assign, 2=prefer)

Now at least you will be anonymized amongst the other nodes of your local network.