There appears to be no way (when using the Debian Installer) to specify
*both* a static IPv4 address (along with netmask and gateway) and a
static IPv6 address (along with netmask and gateway).

It would be nice if when you pick 'Configure network manually' that it
asks you for...
- IPv4 IP
- IPv4 netmask
- IPv4 Gateway
- IPv6 IP (this is missing)
- IPv6 netmask (this is missing)
- IPv6 Gateway (this is missing)

And then it'll ask for nameservers... where you can specify both IPv4
and IPv6 nameservers together delimited by spaces.

If this is confusing for some users we could have...
- 'Configure IPv4 network manually'
- 'Configure IPv6 network manually'
- 'Configure IPv4 and IPv6 network manually'

Instead of just 'Configure network manually'

The way the manual reads (see section 6.3.1.5.3) ...
https://www.debian.org/releases/stable/i386/ch06s03.en.html

It claims "All combinations of IPv4 and IPv6 [...] are supported" but
isn't specific about how you do this, or if it is limited just to
autoconfiguration alone.

At the moment someone has to pick either IPv4 or IPv6 and then manually
edit their networking settings once Debian is installed.

This really promoting the adoption of IPv6!

As most people (where they have an IPv4 to use) will definitely
prioritise making IPv4 connectivity work... more then IPv6.  So if
forced to enter just one IP address by the Debian Installer, they'll go
with IPv4.

Then we have to *hope* they remember to manually add IPv6 to their
network configuration later on.  If they don't just go straight on to
installing software and other setup.

Small omission...

"This really        promoting the adoption of IPv6!"
              ^isn't

:)

Unless of course I've got this completely wrong and there is a way to do
both a static IPv4 and a static IPv6 completely through d-i before the
system even reboots into Debian for the first time.

But if you can, it's certainly not obvious and that's an issue in itself.