> Should it default to IPv4?

It does already. My reasoning was that, because it had to have a default in order to eliminate the forms' "--------" choice, I might as well pick the more commonly used type. If we don't want it to have a default, then either the forms or the code will be pretty ugly (since the other method of eliminating "--------" looks horribly hackish).

> we should verify that a validation error is raised if we give it a IPv6 address without the ip_type parameter.

Agreed, assuming we can come to a consensus about whether it makes sense to have a default value.