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XSDL 1.0 required unions to have at least two member types. Revisions for XSDL 1.0 changed that to require only one; it won't be a common idiom, but there might be reasons to need to do it. In connection with the definition of the xsd:error type, which is defined as an empty group (so that it has no valid instances), the Working Group decided on 27 July 2007 to remove the requirement that groups have at least one member type. That change has been made in Structures, and a corresponding change must be made in Datatypes. Since the WG has already decided essentially what to do, I'm setting the keyword of this issue to 'needsDrafting'.
On 21 September 2007 the WG accepted a wording proposal (http://www.w3.org/Bugs/Public/attachment.cgi?id=489) which allows for empty unions. The WG determined to leave this issue open until an issue of editorial consistency was addressed. I think this related to the difference between saying that a given sequence-valued property was empty and saying that it was absent, but the minutes of 21 September (http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2007Sep/0004.html) are slightly hazy on the subject. If my memory is correct, the consistency issue is related to that mentioned in bug 3869 relating to the treatment of annotation components, and the two should be dealt with in tandem.
Having examined the relevant parts of Structures and Datatypes, I conclude that no further action is needed here (unless to align the description of {member type definitions} in Structures with that in Datatypes). I therefore make the following wording proposal to resolve this issue: Make no change to Datatypes. Optionally align the wording of {member type definitions} in Structures with that in Datatypes. I'm setting the status of the issue to needsReview to reflect the existence of this proposal.
A wording proposal for this issue (among others) was placed on the server on 4 February 2008 at http://www.w3.org/XML/Group/2004/06/xmlschema-2/datatypes.omnibus.200801.html (member-only link).
The wording proposal mentioned in an earlier comment was considered and adopted today by the XML Schema Working Group. Accordingly, I'm marking this issue resolved. Since the originator of the issue is a member of the WG, the adoption of the proposal by the WG is probably sufficient evidence that the originator is content with the WG's resolution of the issue. But if the editors don't get around to it, it would be convenient if the originator could take the time to shift the status of the issue from RESOLVED to CLOSED. Thanks.