It’s worth noticing it if you are using Opera. I had a difficult-to-solve issue recently.
While playing with the idea of starting to use IPv6 on my main PC, I tested the waters by validating this option in the network configuration of my Windows XP. Unfortunately, Opera is already fully ready for IPv6 but my router box is not. So, the first effect it had is that Opera decided to enforce the use of IPv6 for all web sites that mentioned IPv6 capacity in their DNS (not many of them, but Google and Free.fr my ISP are already in the list). The immediate effect was that Opera tried to connect to them in IPv6 but failed (because the rest of my home configuration is not 100% IPv6). For 99% of all web sites, there was no difference…
It took me several days (and some Googling – using Safari and Chrome) to understand that this is a known issue and to remove the (still) useless IPv6 Windows configuration.
I’ll have to upgrade my router and try again.