Found this page a day late. I was more interested in the general networking and awareness than in Fedora 15 (still on FC9 at home and SL/CENTOS 5 at work).
RIPE has a nice page at [1] where you can see the network performance between test sites. Some (Google, Facebook, etc.) have turned off IPv6 again on their main servers, but you can still see (June 10) June 8 in the strip chart, and you can enter random dates in the matrix display.
Arbor has a measurement page at [2]
Akamai have a nice traffic graph at [3] which still (June 10) shows the traffic bump for June 8.
I later found some information on setting network preferences at [4], using gai.conf (per manpage gai.conf(5) ). There is a gai.conf template with instructions in /usr/share/doc/glibc-common which will allow you to, for example, adjust the relative precedence of IPv4/IPv6 where there are both A and AAAA records for a host.
I have IPv6 enabled on a couple of websites (fairly easy) and enabled IPv6 in sendmail for June 8. Disabled again as I don't have a good reverse record and some mailservers reject that. All test sites that I tried browsing to, worked.
Andrew Daviel, June 10 2011