Very often I run across some documentation which is to be produced for various viewing formats by a makefile calling a XML/XSLT processor. Many times the stylesheets themselves access the standard docbook.xsl per the usual URI. Bad thing if you are without connection. I worked around this the last time by finally learning some sed/grep/xargs (ok, one should know that much anyway) and pointing to a local docbook.xsl (they are quite abundant on every system I came across for the last years) but I think it is really unsatisfying, given the verbose nature of XML that there is no better or standard fallback mechanism for this problem. Is there?
If supported by your XML parser, using catalogs can help, see http://xmlsoft.org/catalog.html for xsltproc/libxslt.