Diederik,
Thanks so much for your reply and it sounds as though 0.46 will be quite a release of inkscape with the basic snapping you mention and also the PDF import!
After you mentioned the use of existing libraries, I remembered that the "open source" qcad (more cad oreintated than I need) supports some forms of snap, including the "midpoint" and "intersection". I'm no programmer, but I wondered if it might be possible to reuse these algorithms?
I've taken a look online, and found a screenshot of the "snapping" on autocad (although too heavy for inkscape use, you may be interested to see); the cursor jumps and changes from the standard "crosshairs" to one of the shapes shown when that particular constraint is appropriate:
http://www.cadtutor.net/acad/acad2ki/osnap/images/d-osnap1.gif
Again many thanks