After installing Inkscape 0.47 pre4, we found that extensions don't work and give the infamous lxml error message:
The fantastic lxml wrapper for libxml2 is required by inkex.py and therefore this extension. Please download and install the latest version from http://cheeseshop.python.org/pypi/lxml/, or install it through your package manager by a command like: sudo apt-get install python-lxml
Printing out the real exception, this was due to dlopen not working. I investigated a little bit, and it seems that lxml doesn't work with the python 2.6 shipped with Snow Leopard. The python 2.6 binary is a 'Mach-O fat file with 3 architectures', while the etree.so shipped with lxml is a 'Mach-O fat file with 2 architectures'. I suspect that the 2.6 binary is running with the x86_64 architecture, which isn't present in the etree.so library.
As a workaround, I symlinked /usr/bin/python to point to python 2.5 (which is a 'Mach-O fat file with 2 architectures'), and that makes the extensions work again.
I've seen a few other bug reports for extensions, but none of them seems to describe the exact problem we encountered. In particular, the workaround in bug #345176 does not work.