I've been playing with FrameMaker 5.5.6 this morning, which includes 
a "Save as XML" function. The XML code it produces is well-formed and 
appears to be 100% compliant (no parse errors in most tools). 
FM outputs a CSS stylesheet (I suppose XSL would have been too much 
to ask), but the code display in IE5 Beta PR2 is very good.
 
Of course, while FM creates perfect XML code for linked images: 
<IMAGE xml:link="simple" href=" ... " show="embed" actuate="auto"/>
IE5 hasn't got a clue what to do with this.
Even cross-references are handled nicely, FrameMaker outputing code 
of this form:
<A href="document2.xml#id(78222)" xml:link="simple" 
show="replace" actuate="user" CLASS="XRef">
but again, IE5 doesn't have much of a clue what to do with this. 
Tables are also, as you could have guessed, a mess. 
Despite IE's deficiencies, FrameMaker 5.5.6 is definitely a giant 
step forward in the right direction.
Simon North.