I believe I've got the SVG blanking problem resolved.
Some files may still have blank renderings left over; use ?action=purge on the image description page (on Commons if it's a Commons image) to rerender them.
The new batch of machines set up a few weeks ago ended up with a broken version of libxml2 that ships by default with Fedora Core 4, which is unable to handle an odd but legal XML construct in SVG files exported from Adobe Illustrator.
We'd worked around this problem on the other servers by upgrading libxml2 to the then-latest version which handles it, but apparently this didn't make it into the setup scripts (or else didn't get installed properly by them) on the newest machines.
I've upgraded the laggards to libxml2 2.6.23, the same as is on the older machines, and recompiled librsvg. At least in a test environment, every server in the farm now properly renders the affected files.
There are other rendering issues which could be improved by upgrading to librsvg 2.15.0, but it has dependencies and I've had limited success getting RPMs built for everything such that it actually works so far.
-- brion vibber (brion @ pobox.com)