We've just discovered that XML files exported by Projectlibre cannot be imported directly by MS Project 2010. Taking a look at the generated XML, I noticed that it contains a <SaveVersion> element which is set to 9. However, consulting the XML format documentation on MSDN (http://msdn.microsoft.com/en-us/library/bb968654.aspx) shows that is was only introduced in Project 2007 and has only one possible value: 12 -- corresponding to Project 2007. A value of 9 would not appear to make sense.
After we manually changed the element value to 12, it could be succesfully imported into MS Project 2010.