Mark,

To expound on Brock's submission, there's a great article on schema  
validation written by his cohorts, Aaron Skonnard and Dan Sullivan.

http://msdn.microsoft.com/msdnmag/issues/03/07/XMLSchemaValidation/

The article's offered code is almost production-ready.

I come from the other side of the fence from Dan, so here's my $.02.  All  
of my Web services are developed WSDL-first; this has a few advantages  
that outweigh the time it takes to learn how to do it.

-- Interoperability: This is nearly a non-issue as long as you work with a  
Web services stack that supports the whole W3C XML Schema specification.

-- Security: Using the code that comes along with the article above, you  
get data validation by having the ability to assign facets to data types  
using XML Schema that would take a tiresome number of lines of code and  
would be a pain to maintain.  Regular expressions in schema are powerful!  
Without any kind of validation, either schema- or code-based, you are  
asking for buffer overrun or denial-of-service attacks.

-- Productivity: By agreeing on a WSDL contract before code is written,  
both the producer and consumer of the Web service can work simultaneously.

-- Analysis/Design: You get to design the interface before any code is  
written, and have a contract that can be presented to business users with  
tools such as XMLSpy.  (Its visual representation of WSDL and schema  
rocks.)

A few companies have made it easier to design your Web services in this  
fashion.  My personal favorite is Cape Clear's SOA Editor.  It's free!

http://www.capescience.com/soa/index.shtml

HTH,
Vern