Why we don't want to do any NAT with IPv6
In a comment on yesterday's entry on our IPv6 DNS dilemma, Pete suggested that we duplicate our IPv4 'private address space with NAT' solution in IPv6, using RFC 4193 addresses and IPv6 NAT. While this is attractive in that it preserves our existing and well proven architecture intact, there are two reasons I think we want to avoid this (possibly three).
The first reason is simply that NAT is a pain from a technical and administrative perspective once you're working with a heterogenous environment (one where multiple people have machines on your networks). A firewall configuration without NAT is simpler than one with it (especially once you wind up wanting multiple gateway IPs and so on), and on top of that once you have NAT you start needing some sort of traffic tracking system so you can trace externally visible traffic back to its ultimate internal source.
(There are other fun consequences in our particular environment that we would like to get away from. For example, people with externally visible machines can't use the externally visible IP address to talk to those machines once they're inside our network, because the NAT translation is done only at the border.)
The other reason is political. To wit, the university's central networking people aren't very fond of NAT. Among other things, they want to be able to directly attribute network behavior to specific end devices and possibly to block those end devices on the campus backbone. They will be much happier with us if we directly expose end devices via distinct IPv6 addresses than if we aggregate them behind IPv6 NAT gateways, and the vastly larger IPv6 address space means that we have basically no good reason to NAT things.
(The potential third reason is how well OpenBSD IPv6 NAT works. I suspect that IPv6 NAT has not exactly been a priority for the OpenBSD developers.)
Note that in general the source hiding behavior of NAT has drawbacks as well as advantages; to put it crudely, if outsiders can't tell you apart from a bad actor you'll get lumped in with them. In our environment, avoiding this (with no NAT) would be a feature.