I don't think XML/text archives should attempt to guarantee that floating point types that are denormalized or inf or nan, in any form, are to-the-bit identical after a trip through one of the the serialization library's text archives (and it should guarantee only that zero is still zero). It's a text archive, you get a text representation, and there is no standard for text representations of wacky floating-point types. Fullstop.
One reason you would want to write an XML archive is to be able to play with it with tools independent of boost::serialization. This means you will need to be able to understand the text representations, and since no standard exists, the representation may not be too complicated, as that would be a hassle maintenance wise.
Actually, there is a standard for floats in XML. See http://www.w3.org/TR/xmlschema-2/#float. Of course this doesn't mean that any particular C++ implementation is able to support these values, but it does suggest the way to represent them in XML files when the implementation does support them. Incidentally I recommend the Clinger paper referenced there to anyone interested in external representation of floating point types. I also recommend The Steele et. al. paper referenced at the end of the Clinger paper.