> I'm putting together a web site devoted to Ruby and XML, and plan 
> to include
> a listing of available Ruby packages for processing XML.  Most of the
> information would likely come from the RAA, but I would like to include
> details that currently aren't part of the RAA listings.

Are you going to write a book on Ruby and XML ;-)

BTW:  Great book on VB and XML

> In particular I want to list dependencies, and an explicit notice of
> whether or not any native-code libraries or compilation is required.
> 
> Two questions: Does anyone know of an existing XML syntax for
> describing software (e.g. author, OS, version, dependencies, ...),
> and would it be possible to have the RAA listings include a few 
> more details in the main listing?

I don't know of any specifically, but maybe start with the Dublin Core and extend for source code specific (OS, etc) tags?

see:
http://www.dublincore.org/documents/dces/