Here is how to reproduce this bug:
 * Download http://fgouget.free.fr/bugs/testdep.tar.bz2
   This is a small archive that contains a set of test, empty, Debian
   package files along with their source.
   (run ./build to rebuild them)

 * Install the testdep-foreign_1.0-1_amd64.deb package.

 * Then run:
   qapt-deb-installer testdep-foreign-detault_1.0-1_i386.deb

qapt-deb-installer will refuse to install that package, claiming that:

    Status: Error: Cannot satisfy dependencies

Yet testdep-foreign-default has 'testdep-foreign' as its sole dependency. Given that the 'testdep-foreign' package is a multiarch=foreign package it can satisfy dependencies for all architectures and thus should satisfy the testdep-foreign-default dependency. Running dpkg confirms this:

$ sudo dpkg -i testdep-foreign-default_1.0-1_i386.deb
Selecting previously unselected package testdep-foreign-default.
(Reading database ... 214865 files and directories currently installed.)
Unpacking testdep-foreign-default (from testdep-foreign-default_1.0-1_i386.deb) ...
Setting up testdep-foreign-default (1.0-1) ...

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: qapt-deb-installer 2.0.0-0ubuntu1
ProcVersionSignature: Ubuntu 3.8.0-26.38-generic 3.8.13.2
Uname: Linux 3.8.0-26-generic x86_64
ApportVersion: 2.9.2-0ubuntu8.1
Architecture: amd64
Date: Sat Jul 27 00:00:33 2013
InstallationDate: Installed on 2013-04-29 (88 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
MarkForUpload: True
SourcePackage: qapt
UpgradeStatus: No upgrade log present (probably fresh install)

Reproducible: Always