Hi all,
In his book "System Architecture with XML", Berthord Daum lists some of the DTD deficiencies and thereafter introduces XML Scheme to resolve such issues. One of them is as follows:

Bags: DTDs cannot specify unordered sequences of elements (bags). For a given model group (e1,e2,e3) the elements e1 . . . e3 must appear in the document instance in the defined sequence. To simulate an unordered sequence, all possible permutations must be given as alternatives: ((e1,e2,e3) | (e1,e3,e2) | (e2,e1,e3) | . . . ).

Well it seems this is not resolved in xsd either:
   http://www.thescripts.com/forum/thread86178.html

Is it so ?!

Yes, XML Schema has restrictions in this area too. They aren't precisely the same as the restrictions in DTDs, but fairly similar.

You can use xs:all to say "each one of the following elements may appear zero or one times [you can say which], in any order". You can also use xs:choice to say "each one of the following elements may appear any number of times in any order". What you can't do is to say "there can be up to three authors, up to two editors, and exactly one publisher, in any order".

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference