Why is it giving me:
(...)
INFO: Mapping resource: EquipProdCategoria.hbm.xml
Oct 21, 2003 4:12:05 PM net.sf.hibernate.util.XMLHelper$ErrorLogger error
SEVERE: Error parsing XML: XML InputStream(52)
org.xml.sax.SAXParseException: Attribute "property-ref" is not declared for element "one-to-one"

The "property-ref" is there (below):

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" >

<hibernate-mapping>
<class
name="pt.comseal.arsol.hibernate.EquipProdCategoria"
table="equip_prod_categoria"
>
<id
name="id"
type="int"
column="id"
>
<generator class="assigned" />
</id>

( ... )
<one-to-one
name="equipProd"
class="pt.comseal.arsol.hibernate.EquipProd"
outer-join="auto"
property-ref="equipProdCategoria"
/>

Looks like you are using Hibernate 2.1, but you are for some reason pulling the DTD from the website. The website DTD will not be updated until Hibernate 2.1 is released. Anyway, you shouldn't really be validating against the website DTD.