e.g. mozilla-thunderbird-bin needs libexpat.so.0 to work correctly and also some packages build from source are missing this special file when configuring themselves. Seems, with latest expat, this file isn't created anymore, only libexpat.so.1 and some others. Dirty but quick workaround for me was creating a symbolic link from libexpat.so to libexpat.so.0.
Steps to Reproduce:
Update to newest stable libexpat
Try emerging e.g. kde-base/kdenetwork-3.5.7-r1 (stops while configuring) or try starting thunderbird-bin on the command promt (for getting the error and no running instance)
Emerge completes or application starts
Could confuse some people, because expat is installed but configure or apps can't find it.
Forgot to mention the version: dev-libs/expat-2.0.1
*** This bug has been marked as a duplicate of bug 128069 ***