The attached patch against trunk adds support for the miter_limit property, something that will be supported by pretty much any graphics API, to the stroke class and LineSymbolizer.
Unfortunately it is not complete as I have not yet figured out how to route the property through from the Python API or the XML config.
[springmeyer] background on this is at http://firstname.lastname@example.org/msg03666.html
old patch at http://18.104.22.168:8000/mapnik_trac/ticket/786
8ab8f7c - add stroke-miterlimit processing for line_symbolizer (grid and cairo backends)
NOTE: line_pattern_symbolizer defaults to 4.0