I beleive that the current draft specification
(http://spf.pobox.com/draft-mengwong-spf-01.txt) handles the IPv6 mechanism
adequately, infact I've just been testing that very area with my
implementation.
I've run into some issues with PTR & IPv6 mainly around my inability to get any
sensible reverse record setup which is probably my misunderstanding more than
anything else, but the "a" & "mx" mechanisms already handle the IPv4 & IPv6,
and by using the dual-cidr-length modifiers, you can control the range of
addresses.
-Gary