🚀 go-pugleaf

Rusty, 

What you need is an XSLT processor. Check out Saxon, XT, and Xalon. These are stand alone processors that would be operated from a command line or driven by another application. I recommend getting into a commercial XML development tool instead of a stand alone processor if you just need to do an occasional transformation. It'll have a GUI and have many useful features. I use XML Spy from Altova. XMetal from SoftQuad is also popular. There are lots of great books out there on XSLT. I'm not a programmer but have learned to do my own transformations from XML to xHTML. 

I think you'll find that XML output from InDesign needs a fair bit of massaging to get to valid xHTML but if you have a repeatable process much of the work can be automated. You might want to check out the scripting capabilities of InDesign. Most XML features are scriptable. 

Phil Redman