Edit the xReporter sitemap.xmap file, and add a parameter "locale" with
value "en" to the HSSFSerializer, like:

<map:serializer mime-type="application/ms-excel" name="xls"
     src="org.apache.cocoon.serialization.HSSFSerializer">
  <parameter name="locale" value="en"/>
</map:serializer>

I don't remember having to do anything like this... Was that just a local misconfguration, or is there something we should be doing that we're not? Or has it been resolved already?

I've recently seen this behaviour as well.
It occurs when the xreporter-cocoon instance is running in a JVM that has it's default-locale different from 'en', e.g. 'nl'.

What happens is that the decimal point (dot versus comma) is not recognised as such, and the number gets not interpreted correctly.

Since the xreporter-xml creates the data with decimal 'dot' we should consider adding this locale='en' paramater in our sitemap.

added patch that solves the problem

Applied the fix in revision 673.
http://svn.cocoondev.org/viewsvn?root=xreporter&view=rev&rev=673
Thx for the patch, Kristof!