How do I count the total number of nodes in an SVG file?

I'm not aware of an existing tool that displays this information (sum of all nodes) for all objects/paths in a drawing.

What I would do to get a node count (using Inkscape 0.48):
1) make sure that all objects are regular paths (convert shapes, path effects, text objects etc. to path if needed)
2) select all objects ('Ctrl+A' or 'Edit > Select All')
3) switch to the node tool
4) select all nodes ('Ctrl+A' or 'Edit > Select All') -> since the new node tool allows multi-path editing, all nodes in all selected paths are highlighted (selected)

-> the status line tells the total number of selected nodes.

Note: if there are many paths (with lots of nodes) in the drawing, selecting all nodes in the node tool can take a while, the same applies to deselecting them again. Also: don't save the document if you converted objects to path you'd like to keep as shapes, path effects or text objects).