Comment 20 for bug 845136

I can confirm that removing/renaming the multiarch file will cause lots of breakage. I got lots of problems with various libraries, mostly kerberos-related stuff.
Deleting these packages would have deinstalled half of Ubuntu, so I force-deleted them (leaving their dependencies installed), then reinstalled them. At that point, I'd get the conflict with the i386 versions of the packages that was mentioned above, and would be unable to configure them (rendering them essentially nonfunctional I guess).

Restoring the multiarch file would then delete a huge list of :i386 packages, configure the packages, and restore everything to working order - including the proprietary Flash plugin that had been broken since the multiarch experiment.
So I guess I'll live with the "Forget New Packages" problem and hope it's really just cosmetic - removing multiarch was definitely one of the worst experiments I ever tried on my system.