I was thinking about almost the same, but one objection:
The use of postinstall for messages is flawed. A user installs pacman -S kde and installs all dependencies, all messages scroll over and he lost most of them. It's even worse if you switch to another VT for a while to do something else

On Debian, most of the times I install a package and go to /usr/share/doc/packagename and look for a README.Debian. This file describes the way the package is installed, gives some info, etc. It also gives hints about optional dependencies if you would like those.
This, together with the "Recommends" field dpkg/apt-get has, would be very useful for archlinux. We could even take over the changelog stuff debian uses to list their changes, it's reading commit messages (if any) in CVS and comparing PKGBUILDs to find out what has changed between two versions.

that would simplify the $pkg.install file for some pkgs, where we already echo the people to install pkgs to have some additional functionability.

i personally would not like pacman to ask for installing this additional pkgs while installing the one it's initially run to do.

option 2 (notify only) is the way this should work, if you ask me. also, pacman -Qi should have a part where this info it outputted.

on the method of implementation:
maybe the most uesfull way woudl be to provide the pkgname of the suggested pkg AND an description what this would end in.

suggestdepends=('samba:provides support for Samba in KDE')

or something like this.

if i take an example - k3b:

now it has this:
post_install() {
echo "k3b: If you have a DVD-Burner, you should install 'dvd+rw-tools' "
echo "k3b: For 'k3bsetup' to work, you need 'kdesu'. kdesu part of 'kdebase' "
}

with the new handling it would be like this:

suggestdepends=('dvd+rw-tools:provides support for dvd-burning in k3b' 'kdebase:provides kdesu that is needed for k3bsetup to be used')

an advantage? yes, because the $pkgname.install is then not longer needed and all pkgs with suggested dependencies would be handled the same way across all pkgs. an necessity? not really.

my 2 Rp on the subject

I dont like user-interaction built into pacman. This makes scripting harder/error-prone. But pacman front-ends could use "optionaldepends" for user-interaction.

Juergen

The developer consensus is that optional dependencies aren't really needed.

The situation may change after libpacman comes out. Frontends may get the opportunity to process optional depends.