The jboss_5.0.xsd in the JBoss 5.0GA release (\docs\schema) is invalid.

First of all: The comment in "jboss_5_0.xsd" showing a sample "jboss.xml" file seems to be wrong, as this jboss.xml leads to a deployment error:
This is found in the XSD:
<jboss xmlns="http://www.jboss.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.jboss.org/j2ee/schema
http://www.jboss.org/j2ee/schema/jboss_5_0.xsd"
version="5.0">
...
</jboss>

The deployment error is:
...
Caused by: org.jboss.xb.binding.JBossXBException: Failed to parse source: Element

{http://www.jboss.com/xml/ns/javaee}

jboss is not bound as a global element.
at org.jboss.xb.binding.parser.sax.SaxJBossXBParser.parse(SaxJBossXBParser.java:203)
at org.jboss.xb.binding.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:168)

I found that the following snippet is working on deploy (but version is still invalid, see below!):
<?xml version="1.0" encoding="UTF-8"?>
<jboss xmlns="http://www.jboss.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.jboss.com/xml/ns/javaee http://www.jboss.org/j2ee/schema/jboss_5_0.xsd"
version="5.0">
....
</jboss>

=================
The rest of the problems occur when using Eclipse 3.4/WebToolsPlatform 3.0 to build a "jboss.xml" which references to XSD. The errors also occur when adding the XSD itself to the project so that WTP validates it.

First one:
"Failed to read schema document 'ejb-jar_3_0.xsd', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not <xsd:schema>."

Solution:
Modify import of "ejb-jar_3_0.xsd" to:
<xsd:import namespace="http://java.sun.com/xml/ns/javaee" schemaLocation="http://java.sun.com/xml/ns/javaee/ejb-jar_3_0.xsd"/>

===============
This leads to the next one:
"http://www.jboss.com/xml/ns/javaee":security-identity and "http://www.jboss.com/xml/ns/javaee":security-identity (or elements from their substitution group) violate "Unique Particle Attribution". During validation against this schema, ambiguity would be created for those two particles.

Solution: in the definition of "message-driven-bean" is a dupliate "security-identity" element:
<xsd:complexType name="message-driven-beanType">
...
<xsd:element name="service-ref" type="jboss:service-refType" minOccurs="0"
maxOccurs="unbounded"/>
===> <xsd:element name="security-identity" type="jboss:security-identityType" minOccurs="0"/>
<xsd:element name="resource-ref" type="jboss:resource-refType" minOccurs="0"
maxOccurs="unbounded"/>
<xsd:element name="service-ref" type="jboss:service-refType" minOccurs="0"
maxOccurs="unbounded"/>
<xsd:element name="security-identity" type="jboss:security-identityType" minOccurs="0"/>
<xsd:element name="resource-ref" type="jboss:resource-refType" minOccurs="0"
maxOccurs="unbounded"/>
<xsd:element name="resource-env-ref" type="jboss:resource-env-refType" minOccurs="0"
maxOccurs="unbounded"/>
<xsd:element name="message-destination-ref" type="jboss:message-destination-refType"
minOccurs="0" maxOccurs="unbounded"/>

===> <xsd:element name="security-identity" type="jboss:security-identityType" minOccurs="0" maxOccurs="1"/>
<xsd:element name="security-domain" type="jboss:security-domainType" minOccurs="0"/>
<xsd:element name="method-attributes" type="jboss:method-attributesType" minOccurs="0"/>

Remove one of the duplicates.

Now the XSD validates.
=================
So back to my jboss.xml:

<?xml version="1.0" encoding="UTF-8"?>
<jboss xmlns="http://www.jboss.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.jboss.com/xml/ns/javaee jboss_5_0.xsd"
version="5.0">
</jboss>

Error:
Value '5.0' of attribute 'version' of element 'jboss' is not valid with respect to the corresponding attribute use. Attribute 'version' has a fixed value of '3.0'.

Solution: version must be "3.0":
<jboss xmlns="http://www.jboss.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.jboss.com/xml/ns/javaee jboss_5_0.xsd"
version="3.0">

I will attach a reworked file.

Please take a look at bug https://jira.jboss.org/jira/browse/JBAS-6114
This one is about updating the XSD at http://www.jboss.org/j2ee/schema/ , which would upload the invalid file from 5.0GA, and I think it should be blocked.