@jimp, @johnpoz thanks for the info, I did not know that Windows showed the same behaviour.
I thought it might be an issue, as I had not seen two or three v6 addresses under osx snow leopard (10.6).
Before 10.7, my machines only had the stateless autoconfig addresses.
It seems that 10.7 has silently added a dchpv6 feature, though my dhcpv6 leases page on pfsense doesn't show anything.This blog post
seems to agree with my findings.
Also, while searching for a way to disable stateless autoconfig as per @johnpoz's suggestion, I came accross a post on the same site as above
explaining how to disable stateless autoconfig on windows xp, vista, 7 and osx 10.7 lion.