The most confusing thing is that the PNG is not really opened (Inkscape is not a bitmap editor), but imported. IMHO, the created document should, when a PNG is *opened* directly, use the bitmap size and have a standard "New document.svg" name. Thus it would be saved as SVG by default, and not as PNG.
Confirmed on Ubuntu 10.10, Inkscape 0.48.
The most confusing thing is that the PNG is not really opened (Inkscape is not a bitmap editor), but imported. IMHO, the created document should, when a PNG is *opened* directly, use the bitmap size and have a standard "New document.svg" name. Thus it would be saved as SVG by default, and not as PNG.