Something that I have had in my head for a long time and will probably never actually make it a reality.
I found suitable hardware that could be used in such a project:
http://wwwd.amd.com/catalog/salescat.nsf/doclookupweb/25918BEBCC7071CF862572970038C606?OpenDocument&...Combined with maybe a 40watt panel and maybe 17ah deep cycle battery it could work. Of course to be a truly solar powered server my existing router would also need to be powered by the panel.
Definitely a project for some time next year.