alpm_pkg_get_filename doesn't use the FILENAME field from the sync db.
That's probably because this field didn't exist originally, it was apparently added with 3.0 :
http://projects.archlinux.org/git/?p=pacman.git;a=commitdiff;h=986409f9bd7b84e63352b9ec1f825b0c917627a6#patch3

So I guess there was a transition period where this new filename field could not be used to stay backward compatible with old database.
Since this appeared in 3.0, and we are now at 3.1, I think it's safe to assume the filename field is present now.
Especially because the way filename are re-computed is totally bogus, and doesn't work with any database.

The get_filename function returns "name-version(-arch).pkgext"
It only uses arch when it's available. However, arch field is not available in core/extra database, but the arch does appear in the filename.

egrep -A1 "FILENAME|ARCH" /var/lib/pacman/sync/extra/rxvt-unicode-9.02-1/desc
%FILENAME%
rxvt-unicode-9.02-1-i686.pkg.tar.gz

So here, the filename constructed (the one printed by Sp) is rxvt-unicode-9.02-1.pkg.tar.gz , which is wrong.

And with the community database, it's the opposite : the arch field is in the db, but doesn't appear in the filename! So it's broken here too.

egrep -A1 "FILENAME|ARCH" /var/lib/pacman/sync/community/nexuiz-2.3-1/desc
%FILENAME%
nexuiz-2.3-1.pkg.tar.gz
--
%ARCH%
i686

And get_filename returns nexuiz-2.3-1-i686.pkg.tar.gz

This problem only shows up on -Sp operation and not -S, because during an -S operation, the package metainfo are loaded with the DESC level before get_filename is called.
And the DESC level is likely required because the frontend prints the package size before starting the download / installation. So there is nothing broken / strange here.

To sum up, it's not possible to guess the filename field from the name, version and arch fields, so this bogus code should be simply removed, since we can use the filename field from the db directly now, and everything will be fine :)

This was further discussed on the ML, and finally, the filename field can't be always used, since it's not always present (there are still many old db entries without it) :
http://www.archlinux.org/pipermail/pacman-dev/2008-February/011136.html

So a more conservative fix was included for 3.1.2 :
http://projects.archlinux.org/git/?p=pacman.git;a=commit;h=b206aaee881abd6e256428f2146212cfc1c69be2

If we really want to totally clean this, we would need to regenerate all official databases (core,extra,testing,unstable) so that all entries include the filename field.
I'm not sure how to proceed with this however. Dan or Aaron ?

Xavier, you know my position on the "official" DBs. :)

Marking this as a 3.1.2 bug, and we know the actions we need to take to further resolve it, so we'll handle those on the ML.