If you blacklist ipv6 (which I tried) then some installed software, and some interfaces will belch out error messages, such as:
May 26 21:13:19 localhost kernel: bridge: Unknown symbol ipv6_dev_get_saddr (err 0)
You might want to look at
/etc/sysconfig/network-scripts/ifdown-ipv6
I always turned it off by adding: NETWORKING_IPV6=off to /etc/sysconfig/network.
Not sure if that will still work on Fedora 15, did fine on RHEL 5.