sys-fs/udev tarballs includes a header file, extras/volume_id/lib/libvolume_id.h. The current ebuilds don't install it, but it is needed to compile, for example, the latest versions of Hal from git - so this is going to become an issue once a new version of hal is released.
Created attachment 95432 [details, diff] patch for udev-096-r1.ebuild Suggested patch to alter udev-096-r1.ebuild to install this header.
Am checking with the upsteam developers about the status of this...
Will be fixed in the 098 release.