From the XSD 1.0 spec on w3.org:
The XML representation of schema
components uses a vocabulary
identified by the namespace name
http://www.w3.org/2001/XMLSchema. For
brevity, the text and examples in this
specification use the prefix xs: to
stand for this namespace; in practice,
any prefix can be used.
in the end xs or xsd are only prefixes. XSD is used for example more by Microsoft schemas.
The important is how you declare the namespace.
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
...
</xs:schema>
or
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
...
</xsd:schema>
should be equivalent.