IPv6 SPF Records

[eric-list-ipv6-ops at catastrophe.net]

I've noticed some SPF-clueful mail servers are denying mail sent from an
IPv6 source address. Has anyone else been bumping into this issue if your
mail servers are v6-enabled? If so, what did you do? Are there any
guidelines for configuring SPF records for IPv6 netblocks? I've looked in
the spf documentation, but assume this may be one of those "hidden
configuration" details. :-) 

Thanks.

- Eric