Hi,
During this GSoC a LibreOffice student and her mentor implemented support for VSD, binary Microsoft Visio documents, that Microsoft never bothered to publish a specification on. The student managed to do far more that she expected and ended up with a nice C++ library for parsing VSD called libvisio that supports all geometric primitives, formatted text, embedded stencils, all kinds of fills and strokes etc. The long story is here: http://libregraphicsworld.org/articles.php?article_id=41
Now, the short story is that the libvisio library (currently at v0.0.7) has a tool called vsd2xhtml which converts VSD to SVG documents and embeds them into XHTML documents. So it looks like it's possible to write an extension for loading VSD directly to Inkscape. Otherwise you have to do some text editing to get an SVG out of that XHTML.
Howevere there is a party trick involved. Since VSD files are often multipage, the convertor dumps every page into same XHTML file one by one. So if someone decides to have a go at it, I think it would rather be loader similar to our PDF loader which provides preview for letting a user to choose a page for importing.
Alexandre Prokoudine http://libregraphicsworld.org