> Is it the case that a schema-aware processor processing an 
> untyped input
> document behaves the same as a non-SA processor on that document? 

Yes. A basic processor differs from a schema-aware processor only in that it
rejects certain things that an SA processor accepts. If you're within the
subset that a basic processor can handle, then the two give the same
results. 
> 
> 
> > > -Is it possible to store the validated source (the PSVI?) 
> > and supply 
> > > that for subsequent transforms?
> > 
> > Watch this space....
> 
> 
> I think that's the clincher at the moment.
> 

I think the other important thing for many people will be the ability to
store a pre-compiled schema persistently. For that, you'll have to watch
this space a little longer. It's OK, of course, if you're in a
continuously-running server environment, where source documents, compiled
stylesheets, and compiled schemas can all be cached in memory today.

Did you know section: did you know that if you cache source documents and
stylesheets in memory, then Saxon will also cache the indexes used to
support xsl:key, which will stay in memory so long as both the source
document and the stylesheet are in memory? (A long-standing item on my to-do
list is to use a common index if two stylesheets define the same key
definition!)

Michael Kay
http://www.saxonica.com/