After installing Fedora Core 5 and setting yum to use the gst repositories as specified in http://gstreamer.freedesktop.org the videoscale plugin will not load and "gst-inspect videoscale" will report that this plugin is not installed, despite it is present as a file. gst-inspect also reports the following when issuing as root / user:

GST_DEBUG=*:3 GST_DEBUG_NO_COLOR=1 gst-inspect-0.10

result:

[...]
WARN  (0x88a78f8 - 0:00:00.550508000)   GST_PLUGIN_LOADING( 2725) gstplugin.c(410):gst_plugin_load_file: module_open failed: /usr/lib/gstreamer-0.10/libgstvideoscale.so: undefined symbol: oil_function_class_ptr_merge_linear_u8
[...]

As a result all videoscale - related gstreamer calls do not work. This is resolved if liboil is upgraded from 0.3.6 to 0.3.8. Fedora Core 5 users who have not issued a yum update (or any other system that is using this particular liboil version) could be affected.

To sum up, it is better to always check liboil version before issuing yum install gstreamer-universe. "yum update liboil" can be of help if you want to get things up and running fast.

My thanks to all #gstreamer devs + users over irc.freenode.net who provided a direction for solving this.