My first thought is to use
<para>foo</para>
as this was available in VS2008 for the comments http://social.msdn.microsoft.com/Forums/en-US/5dce48f5-d810-46bd-bb9b-6d0478174cf2/xml-comments-how-to-insert-a-new-line?forum=csharpide
Have you looked at putting your comment inside a CDATA or similar to try and preserver the whitespace?
http://msdn.microsoft.com/en-us/library/vstudio/ms255811(v=vs.100).aspx
IIRC it's more of a formatting thing for the IDE however, I think a doc generation tool like Doxygen observes the whitespace