Summary and Info:
Certain custom packages, such as kernel modules, require rebuilding after updating another package, such as the kernel. Unfortunately at present there is no way to "link" the two in pacman's db.

Steps to Reproduce:
An example I've come across is virtualbox_bin in the archlinuxfr repository though I'm sure there are more. The post-installation notes for virtualbox_bin specifically say:
> IMPORTANT NOTES:
>
> - Run "vbox_build_module" as root every time your kernel is upgraded, to
> compile the module for the new kernel version.

Proposed Solutions:
Add either 1 or 2 new fields, similar to depends=() to aid in automating the above manual process. Unless you *remember* that you installed package A that requires commands to be executed when package B is updated, you might only notice issues after you've irreparably damaged important data (worst-case) or you've been battling to figure out why your application A isn't working (typical case).

For now my thoughts are as follows:

Solution A
Run a command / series of commands automatically

addtoinstall=(kernel26)
addtoinstallcommand=vbox_build_module

Solution B
Print "special" notes automatically

addnotes=(kernel26)
notestext=/opt/virtualbox/rebuild-instructions.txt

Solution C
*Advise* reinstallation of package
reinstalldepends=(kernel26)

After kernel update, ask:
archlinuxft/virtualbox_bin advises reinstallation after update of kernel26. Proceed? [Y/n]

Personally, I'm in favour of C as it is the simplest and probably the most reliable. It *may* not fit all cases.
B is the least "dangerous" and would at least give an appropriate warning when doing updates. It is not foolproof however.
A is probably the most direct solution.