I am about to build a piece of a project that will need to construct and post an XML document to a web service and I'd like to do it in Python, as a means to expand my skills in it.  
Unfortunately, whilst I know the XML model fairly well in .NET, I'm uncertain what the pros and cons are of the XML models in Python.  
Anyone have experience doing XML processing in Python? Where would you suggest I start? The XML files I'll be building will be fairly simple.
I've used ElementTree for several projects and recommend it. 
It's pythonic, comes 'in the box' with Python 2.5, including the c version cElementTree (xml.etree.cElementTree) which is 20 times faster than the pure Python version, and is very easy to use.
lxml has some perfomance advantages, but they are uneven and you should check the benchmarks first for your use case.
As I understand it, ElementTree code can easily be ported to lxml.
XML Processing in Python