I’ve been using http://odf-validator.rhcloud.com is it updated? It reports that Writer 3.5 files are invalid, with a few attributes that aren’t expected.
I’ve been making tests with Writer 3.5 and Word 14 and trying to build my own template, even editing XML directly.
There are invariably going to be errors if you are using the default (ODF 1.2 Extended) because this includes elements and attributes that are in addition to plain ODF 1.2 i.e., used in an interim manner until ODF 1.3 is available. You can set the version under Tools > Options… > Load/Save > General > under the Default file format and ODF settings section.
Probably the best is http://odftoolkit.org/projects/conformancetools/pages/ODFValidator the Apache ODF Toolkit.
I assume that tool is up-to-date (it include all new variates of standard), but for sure of yourself You could ask authors.
I edited the answer to point to the new page that the old link attempts to redirect to, as the original link was dead. Unfortunately though the link to the source code at the foot of the new page is also dead. Source code can found here.
The OpenDocument Fellowship has a validator that embeds the validation engine provided by Cyclone3. It only appears to support ODF v1.0. There is some information on this OASIS page on how to go about validating an ODF document. The information does however appear similar to that given on the Apache ODF Toolkit page.
There’s a list of ODF Validating Software on the TDF Wiki.
One of the newer entries on that list is the “Office-o-tron”, a java-based command-line validator that can handle both ODF and OOXML documents.