Visual Studio solution file isn’t Xml – level 100

I had to manually edit a Visual Studio 2003 solution file today.  I changed the source control location for the contained projects, and I found it easier just to edit the solution file than to use the GUI to fix it.  One thing I found particularly odd is that the solution file isn’t Xml.  Project files are Xml, but not solution files. 

Whidbey doesn’t make solution files Xml either.  I wonder why a custom format would be chosen over Xml.  With backward-compatibility being such a high priority, I now wonder if the solution file with _ever_ be converted to Xml.